You can use the FindResource and LoadResource functions to "load" the resource, and then use LockResource and SizeofResource to get the address and size of it, which you can then use in a BASS_StreamCreateFile call. Something like this...
HRSRC rsrc=FindResource(NULL, resourcename, RT_RCDATA);
HGLOBAL global=LoadResource(NULL, rsrc);
HSTREAM stream=BASS_StreamCreateFile(TRUE, LockResource(global), 0, SizeofResource(NULL, rsrc), 0);