If you could get RTS working then you could reconstruct the data into a screenshot.
Alas the RTS function never made it across from the EZ3.
To the best of my knowledge no GBA flash cart got actual screenshot features though.
That said if every modern GBA flash cart ends up with several hundred K of save memory and things have long been hacked into games like BOMA for soft reset you might be able to work something that way. It would be a horrible kludge (and I would just say copy the normal save over and use an emulator) but it could work.