Placing Wedge and Gestahl in 2 extra char slots through events

Heh, seems like Wrexsoul and I are somehow alike?

Semi-playable characters like Banon, Wedge/Biggs and Leo have this special coding that prevents the player with tampering with their equipment (there for keeping the player from obtaining, say, an offering (Master's Scroll) early in the game)
I theorize (I know basically nothing about this but...) that there is some sort of blocking code that prevents the equipment from being changed. Remove that, and that should be it. If not, there is a smaller byte code that says the characters can only have this, and also prevents changeable equipment. If this is the case, then it's a simple matter of copy and pasting the code from a playable character to the semi-playable character.

I agree with this one, there is some code in the game(C3 bank) that specifies that all characters up to Gau are equippable (including Gogo). Umaro and the rest's equip and relics menu aren't accessible at all. Just change two or three bytes around.

As for making them fully accessible & playable through a character selection screen 'n all, well, that's basically the equivalent of wanting to fight a gun battle in the times of Rome, it's not impossible (they had the materials to do it) but it's VERY hard to accomplish, and likely won't be accomplished successfully until a group of ragtag hackers disect the rom byte by byte. Until then, good luck, and if you do it, then you have my undying loyalty and utmost respect.

I was thinking that maybe this is possible if these characters are recruitable in the same way as shadow or gau are recruited (having only three party members). For example, have Biggs be recruitable in Gogo's Cavern, Leo recruitable around his grave, etc.

The problem would be that if you ever dare switch party members in the falcon or somewhere else these characters may be lost. This may be the hard part - Code an event that removes these temporary characters and returns each one of them to their original recruiting places (Biggs to Gogo's cavern, Leo to his grave, etc.) so they may be recruited again later on.

Kefka has no commands and all his stats (speed, strength, stamina, etc) are 80 by default. To make him even more awesome, he comes equipped with a paladin shield. If you make him obtainable, the game will become boring and easier, if you ask me.