I know it sounds like I'm not any fun, but
I Wouldn't Allow It
Getting close to the sleeping koi killer is easy: The koi avenger makes an opposed skill check (the koi avenger's Stealth skill versus the koi killer's Perception skill). The koi killer takes a -10 penalty to his Perception skill check because he's sleeping. If the koi killer wins, he awakens. If the koi avenger wins, the koi killer's asleep.
But then nothing I can find short of mind control--be it with the skill Sleight of Hand or otherwise--permits the koi avenger to make the koi killer--awake, asleep, or unconscious1--take something from him, be it something beneficial (e.g. ioun stones), malicious (e.g. pariapt of foul rotting), neither (e.g. girdle of opposite gender), or some combination (e.g. bag of devouring). The game just doesn't allow forced-equipping. The koi avenger could drop the girdle on the koi killer from a great height for possible damage. The koi avenger could lie to the koi killer about the girdle's provenance and give him it... or sell him it. The koi avenger could even stash it among the koi killer's unattended goods.
But, according to the rules, the koi avenger can't strap the girdle onto the koi killer without the koi killer's permission.
A DM can house rule this, of course. It could be this simple: The DM allows the koi avenger to take a full-round action to make an opposed skill check (the koi avenger's Sleight of Hand skill versus the koi killer's Perception skill). The koi killer takes a -10 penalty to his Perception skill check because he's sleeping. If the koi killer wins, he notices the attempt and the koi avenger provokes an attack of opportunity from the koi killer. If the koi avenger wins, the koi killer doesn't notice the item among his possessions. If the koi avenger beats the koi killer's Perception check result by X or more, it's also as though the koi avenger used the item normally.
But even if X is outrageously high, that's an incredibly dangerous precedent. That's partially because of what's going on in the question ("How can I force foes to wear the clothes I want them to wear?" to which the answer should be, "Use magic to make them"), but also because of one ancient legacy spell, the 8th-level Sor/Wiz spell trap the soul, which reads, "As soon as the subject picks up or accepts the trigger object [for the spell trap the soul], its life force is automatically transferred to the gem without the benefit of spell resistance or a save." Emphasis mine. (Also possibly a concern is the host of spells involving scraps of paper that can then be deposited on foes or by foes (e.g. explosive runes, illusory script).)
You think adventurers are paranoid now, wait until they're surrounded by admirers in the town square yet must burn their clothes afterward--without touching them, maybe while in them--for fear of accidentally touching a triggering object for the spell trap the soul that anyone could've placed in their pockets. Expect careful inventories of every single item every single morning--by scent so as to avoid accidental contact. Expect lots of wasted time. Expect naked adventurers.
Cursed Items and Vengeance
Cursed items are funniest when users suffer because they are careless, greedy, reckless, or whatever. A cocky adventurer is supposed to find the girdle, foolishly assume it's a girdle of giant strength eminently suitable for his massive form, bully the party into letting him have it, flex a little, strap it on, and--bam!--he's a lady. A good laugh is had by all, and everything's back to normal after the adventurer's the target of an effect like the spell remove curse et al. The adventurer will know better (or not) next time to run some tests on a found magic item or have a magic item identified before he uses it. There's a point to that.
Changing the sleeping koi killer's gender is not revenge. That's a prank, what with it being "generally lighthearted, reversible and non-permanent," and aiming "to make the victim feel foolish or victimised." In fact, I'd even argue it "involve[s] cruelty verging on bullying" as it's being "performed without appropriate finesse." If the koi killer really wants revenge he needs to make it appropriate, do it with impunity, and make sure the koi killer knows that the koi avenger did it and why the koi avenger did it.
I'd advise the potential koi avenger to reconsider and instead just haul off and sock the koi killer, saying, "That's for my fish!" Unless, of course, that fish needs avenging, in which case such avenging should be done correctly and with style.
- Okay, there's this: "A character can carefully administer a potion to an unconscious creature as a full-round action, trickling the liquid down the creature's throat. Likewise, it takes a full-round action to apply an oil to an unconscious creature." But that's more like casting a spell on someone than making them accept an item.