Hi!
I´ve tryed like themadhacker says and actually its works on gba for gbc games, I think it would be ok on gbc too.
But i discoverded 2 things:
1)If you push the change speed button you can run it lowly, really slowly and no crashes running gbc games
2)when running gba games, if you do the change, the game will crash, but if you retain the button pressed it will not and run slowly (like when gbc cart is used). So, the cristal at 8 mhz combined with the original is too much for gba, and maybe need a slowly cristal, i.e. another of 4 mhz or 6 mhz ( i didn´t tryed out) combined with te swich acting as a selector between 4 (gba) or 8 (gbc) crystals
Otherwise, I only save when the gba is running at normal speed. I dont try to save when is running faster and lose my saves, so I don´t know.
Here are pics, the firs one is for the circuit
The second one is the gba with the mod
Thanks to Basement_Modder !!!