If Minsc plans to employ the special ability supreme cleave of the prestige class frenzied berserker (Complete Warrior 34-6), Minsc is out of luck—that special ability is specifically limited to once per round. However, Minsc may have better luck convincing the DM this will work if Minsc is using a different version of the special ability supreme cleave like that of the prestige classes knight protector (55-7) or master samurai (Sword and Fist 29-30 yet sadly rendered obsolete by this document).
Even then, Minsc's player must argue against the Sword and Fist FAQ that includes this exchange:
Is there any limit to how many 5-foot steps a master samurai can make when using the supreme cleave ability? The Cleave feat says you can attack another creature in the immediate vicinity when you drop a foe with a melee attack. What is the definition of “immediate vicinity?” How does that change when using supreme cleave?
A character can take only one 5-foot step each round, and then only if the character has not otherwise moved during the round. Supreme cleave lets you step between cleave attacks, but you still can step only once.
In the case of Cleave, “immediate vicinity” means within melee reach. A character using supreme cleave can first take a 5-foot step to determine who is within melee reach before choosing a target for a cleave attack. (3)
(This is the only official-ish word I'm aware of on the special ability supreme cleave, and, while it's for dnd-3e, the 3.5 revision left the special ability supreme cleave largely unchanged.)
To be clear, the knight protector's extraordinary ability supreme cleave says, "Beginning at 3rd level, a knight protector can take a 5-foot step between attacks when using the Cleave or Great Cleave feat" (CW 56), and the master samurai's extraordinary ability supreme cleave says, "At 2nd level, the master samurai gains the ability to take a 5-foot step before making a Cleave or Great Cleave attack" (SF 30).
So what Minsc's player must do is convince the DM that the special ability supreme cleave overrides the typical limit on taking a 5-ft. step: "You can move 5 feet in any round when you don’t perform any other kind of movement" (PH 144)… and, of course, that the Sword and Fist FAQ is wrong. (Pro Tip: Sometimes FAQs are.)
In some cases, this will be easy: The DM may rule that the special ability grants the creature as many 5-ft. steps as are needed, specifically independent of the existing rules. That is, a special ability enables the creature to do it—regardless of what it is and regardless of the existing rules, the special ability existing essentially as an independent operator, the whole of it a specific exception to the general rules. This is totally valid and enables lightspeed supreme cleave: as long as Minsc never rolls a 1 on the attack roll, Minsc can, by hacking through kobolds, travel from Rashemen to Candlekeep during one attack.
(To clarify further, taking a 5-foot step is no action. A DM can, by the rules, arbitrarily limit the number of free actions a creature can take in a round, but the DM must make a house rule that says there's a similar limit on no action actions.)
However, in other cases, Minsc will find this more difficult. This DM, for instance, rules that the normal rules apply unless a special ability specifically overrides the normal rules—and while some abilities specifically enable multiple 5-ft. steps—the feats Psionic Sidestep (Expanding Your Mind Web column "New Psychic Warrior Feats, Powers, and Ranged Weapon Enhancements") et al., Pursue (Eberron Campaign Setting 58), and Sidestep (Miniatures Handbook 28), for example—, the special ability supreme cleave of the knight protector and master samurai does not, so this DM otherwise hews to the printed rules.
Thus, in this DM's campaigns, the DM sides with the harder rules and the FAQ and the knight protector and master samurai special ability supreme cleave allows only one lone 5-ft. step per round between one attack triggered by the feats Cleave (PH 92) et al. and that 5-ft. step granted by the special ability supreme cleave is limited as normal—the creature can't've taken any other movement during the round, including another 5-ft. step.1 So unless Minsc has some other way to take multiple 5-ft. steps during a round this DM's disabled lightspeed cleave, and Minsc is stuck in Rashemen, and instead of suggesting Minsc use a wish for kobolds sufficient to overrun Mulhorand, Boo should've urged Minsc to wish for—I dunno—, like, a really high kibble mountain and a surfboard or something.
An alternative: lightspeed mounting and dismounting
Instead of using the wish for kobolds, Boo could've suggested using the wish for a line of horses (or some other kind of mount appropriate for Minsc), starting adjacent to Minsc and each horse thereafter 5 ft. apart, stretching from Rashemen to Candlekeep.
Minsc then could've taken a free action to make a Ride skill check to fast mount the adjacent horse (DC 20 but an armor check penalty applies) then take a free action to make a Ride skill check to fast dismount the horse (DC and potential penalty the same) on that horse's opposite side then repeat the process until, in the space of a free action, Minsc reaches Candlekeep.
However, the DM will likely limit the number of free action Minsc can take to fast mount and dismount. (In this case, this DM tends toward a limit like a creature's Dexterity bonus, minimum 1.) Further, this article—a Rules of the Game column, so be skeptical—restricts greatly the fast mount skill use of the Ride skill. But assuming a sufficient Ride skill modifier and using only the core rules and if the DM's extremely generous, a goldfish, or a stunningly lifelike computer simulation, Minsc could fast mount and dismount from Rashemen to Candlekeep in the blink of an eye.
1 That might sound terrible—and it kind of is now—, but bear in mind that all three of the prestige classes frenzied berserker, knight protector, and master samurai—so far as I'm aware, the only official classes that have the special ability supreme cleave—originate from Sword and Fist (Jan. 2001), one of dnd-3e's earliest supplements, and its adnd and adnd-2e roots show: those earlier games often demanded PCs murder hordes of inadequate foes.2 In such situations, being surrounded by such foes—hence unable to move—, killing all those foes, then being able to take a 5-ft. step to kill also the next wave was kind of a big deal, perhaps saving for the next encounter the wizard's fireball.
2 Used to the more conservative number of foes in dnd-3.5e, players in my current campaign that has the PCs venture into classic dungeons have been taken aback at the sheer volume of foes the adventure expects their PCs to face: sometimes, like, twenty or more antagonists all at once… in the Keep on the Borderlands! The players like their characters but they sort of regret not taking the feats Cleave et al., and the special ability supreme cleave—even if it enables only a lone 5-ft. step—is useful in such situations.