More Ultimate Equipment is a 3rd-party book...
... and 3rd-party in 3.5 was notoriously shoddy. That is important to note if your group is planning on houseruling this in either direction, because there is no indication that these are exotic weapons as a blanket descriptor.
Within the book, there are several exotic weapons scattered around (the ice axe on page 27, the fuxina, bisento, and battering axe on page 34, and the sica on page 35). Each of these are specifically called out as being exotic weapons. Myriad weapons do not have this text, and in fact, several of them state what weapon they count as.
Myriad Axe: A myriad axe is a steel axe of hand axe size balanced for throwing. The end of the handle has a rod-shaped intrusion that can be used to fit up to two additional handles end on end. If one more is fitted, the myriad axe gains the statistics of a battle axe. If both extra handles are added, the weapon gains the statistics of a great axe.
Myriad Spear: A myriad spear is much like a myriad axe in that it comes with multiple heads and lengths of handle. In its base form, the myriad spear has the statistics of a javelin. With one additional length fitted to the end of the first one, the weapon becomes a shortspear. Two turn it into a longspear.
Myriad Warhammer: The myriad warhammer is the most mutable of the myriad lot. It starts as a light hammer and comes with interchangeable cold iron, silver, adamantine, and steel striking plates. It also has the same four types as curved spike ends to use either in conjunction with the hammer heads (making the weapon a hooked hammer), instead of the hammer on both sides (making it a pick), or all the heads can be left off to make the weapon a club instead. The myriad warhammer also comes with an extra handle to make it a quarterstaff or a warhammer if the hammer head is left attached. A second extra handle can be added in conjunction with the hook spike ends to make a ranseur.
Emphasis mine. Myriad daggers have no such wording, so I would assume that it works like a normal dagger. I can't say for sure, though, because it doesn't actually say what weapon it is, and none of the other ones stick to the weapon they're named after either. In any case:
- Myriad Axes are either battleaxes or greataxes, so they're martial
- Myriad Spears are either javelins, shortspears, or longspears, so they're simple weapons.
- Myriad Hammers can be
either clubs or quarterstaves (simple weapons), warhammers or
ranseurs (martial weapons), a "pick" (3.5 has a light and heavy
pick, so it might be either. Regardless, this would be a martial
weapon), or a "hooked hammer" (likely the gnome hooked hammer,
which is an exotic weapon).
The only one of these with an explicit exotic function is the myriad hammer, so you should be fine without Exotic Weapon Proficiency unless you're planning on using said hammer as a double weapon.
From a personal standpoint, I don't think that the benefits granted by the myriad dagger warrant a feat tax—you're already being taxed on gold for it, after all.