Got around to trying this out, and it does work very nicely!
Qubits already pointed this out, there's a little oddity to loading savestates in Castlevania Symphony of the Night.
It load it with the correct resolution output, SotN likes to switch the resolution a lot, like 640x480 for video clips, 256x240 for the actual gameplay, and some weird widescreen/letterbox resolution for the pause menu.
So you load a savestate for normal gameplay after loading the game, and you get a tiny image in the top left corner with graphical problems, but is immediately fixed by opening the pause menu because that switches the resolution.
Fixing this should be straightforward, savestates just need to capture that resolution mode and enforce it when you load the savestate.
I'd also like to request a display option. Can hardware fullscreen be implemented like in PocketSNES?
It's my understanding that this can take advantage of the GCW Zero's GPU to draw the image to fit the screen with smoothing, which would be good for all the non-4:3 resolutions.
Things like the pause menu for SotN can't display text properly because the image is reduced without anti-aliasing.
Alternatively, display options that do not stretch a smaller image to fill the screen, so SotN's main gameplay can be displayed 1:1 rather than being stretched wide non-uniform.
Thanks again, love the work you've done.