Hi Jeamsler,
I have done exactly what you are proposing, I have moved my BIAB SFZ files to a second drive.
I just moved the folder with the samples "/Library/Application Support/PG Music Inc/Band-in-a-Box Sounds/Programs".
Although an alias to the new "Programs" folder placed in "/Library/Application Support/PG Music Inc/Band-in-a-Box Sounds" might work, I used a symbolic link instead.
If you are not aware, a symbolic link is similar to an alias but more durable if things change with the path.
I used this free program to create symbolic links, it's easy, and it works really well (no connection to developer).
https://www.macupdate.com/app/mac/10433/symboliclinkerHopefully this helps.
- Jay