Hey Eddie,
I see what you mean about some of those plug-ins being hit or miss with the Mac.
I did notice that most of those sampler instruments at the bigcat site you posted were made with Maize sampler.
Those Maize made plug-ins (including Versilian VS-1 Upright Piano) have a problem with BIAB.
The GUI doesn't show up when using the AU versions, but the VST versions I tried seem to work with BIAB.
Those same AU instruments work fine with all my other AU host programs though.
This is a pain since I mostly use AU plug-ins in Logic, and don't have the disk space to keep 2 versions of the same instruments, which tend to be large.
Hopefully some others will chime in here with their favorite freebies, I can't believe that you and I are the only cheapskates here.

- Jay