If you're using the BASSenc add-on to write the WAV file, you can use use BASS_Encode_AddChunk to add a BWF "bext" chunk to the file, something like this...
TAG_BEXT bext; // BWF "bext" tag structure
// fill the structure here
BASS_Encode_AddChunk(handle, "bext", bext, sizeof(bext)); // pass it to the WAV writer