Surely the texture/model swap can be on a "per cutscene" basis. Like; when the cutscene is triggered the textures swap, then swap back after the cutscene finishes.Problem with that is that all the other cutscenes which do use barrier's texture would use gravity's now.
I need to find precisely where the game determines what model to load for each cutscene, so I can make the proper change. Making a texture change would break other cutscenes. I'm not sure precisely what cutscene the barrier model starts being used, but I think it's after event dm24 that the Varia Suit is activated.
I tried looking around in the files for any kind of calls for cutscenes but didn't have any luck so far.
About your second suggestion, I did attempt doing that at one point. Isn't the scene with the Baby Metroid in sepia tone though?
As for the Mother Brain one, I did try, but damn the lighting and everything else in the scene makes it obnoxiously hard to work with for masking with video editors.
If someone has a way to change Samus' Varia Suit colours in that one to be Gravity Suit, that'd be amazing, I just couldn't do it with Cyberlink's Color Director, maybe there's a better (and free) tool to make it, but I didn't find one.
Or perhaps make the texture/model change for cutscenes after the Gravity Suit is activated. Since the model swaps when the Varia Suit is enabled, surely that can be done.
I'm no good with coding, but if it's already there for when you go from Power to Varia, you should be able to change it when you go from Varia to Gravity.