A few other issues came up:
Running the reverse stream in forward mode through a decoding/resampling mixer, should that be okay?
I noticed position seeks (initial position of a reverse forward mixer source) to fail sporadically, i.e., seek to the wrong position (almost a rounded start position).
Setting a mixtime BASS_SYNC_POS to stop decoding of the reverse forward source (BASS_ChannelStop( source )) does not trigger at the right position (the same position it would trigger at in a regular BASS stream) in the stream, so a BASS_MIXER_END mixer won't end properly. It usually triggers too late, leaving extra audio behind the trigger pos.
Repetitive seeking (e.g., looping of portions) in a reverse stream in reverse mode causes a diverse array of GPFs in bass_fx.dll, and sometimes going deeper down the call stack to msvcrt/ntdll. Easily reproducible by variying the loop portion and seeking back to the new start position (think changing a loop while playing) continously.
All tested with accurate prescans, and mostly on 16/24-bit uncompressed wave files.
Edit: Verified with two compilers, on three different systems.