Costume Room (Idea)

Short answer: yes, given the limitations of the ROM and the original code.

Long answer: probably, but (given my admittedly limited understanding of the graphical system in the ROM) it'll involve a significant amount of ASM hacking, and you run into issues of available space for spritesheets, and the availability of the spritesheets themselves. (Celes' opera outfit, for example, has a fairly limited number of sprites available; only the fourteen playable characters have full spritesheets, as the guest characters like Leo and Banon don't have riding sprites available, although intrepid hackers/spriters have drawn up complete spritesheets for many of these characters.)

Changing the character's overworld palette -- that's what's done for Gerad, for example -- is a simpler endeavour; overworld palettes are set via event. Battle palettes are more complicated; you'll notice that in vanilla, Edgar's battle sprite has his normal palette rather than the modified Gerad colours. There is a hack available for conditional battle sprites that sets Edgar's battle palette to Gerad's during those events, so you have something to work from, at least. (Or whoever you decide to rope in for the ASM work. I maaaaaay take a look at that myself, actually, although I make no promises. ) It may even work as-is; it's been a while since I've looked at that particular code.

TL;DR: changing palettes (Edgar->Gerad) is a much simpler endeavour; full-blown costume changes involves a significant amount more work, and I don't know if anyone's expanded and/or relocated the sprite data to allow for full sheets for more sprites.