I have the same problem, using BASS_Encode_MP3_StartFile seems not to write any ID3v1 tags (when --id3v1-only is set).
Setting --add-id3v2 solves the problem, but only writes ID3v2 tags at the beginng of the file.
And yes, I made sure that all tags are enclosed in escaped quotes using \", but it doesn't add ID3v1 tags at the end of the file...