Can I suggest that you guys add an API like the Edge Mobile Game Engine has?
CFbsBitmap * GetSysApiSurface(void)
You could call overload it for Symbian and call it GetDC() and ReleaseDC(), just have it return a CFbsBitmap *.
It should be trivial since looking in your headers reveals that you're using CFbsBitmap as the base implementation of your CGapiSurface.