Sofar no support for Advanced Sub Station subtitles. However, as from build 808 some basic support for SSA subtitles (bold, italic, underline, font name and color codes) was added. See BSPlayer's
news section (click here) under the information concerning "Version 1.00 RC1 Build 808 released (27 Apr 2004)" (you'll have to scroll down).
I don't know what is in the pipeline at BSPlayer's developer(s), so I can't tell you. (No doubt your post will be read though!)
P.S.: ".ass"-subtitles could perhaps be converted to ".ssa"-format using a subtitle-utility like (e.g.)
Subtitle Workshop or
Winsubtitler (which is the successor of SubMax, the development of which is discontinued), or perhaps by another utility which may be found on the internet (e.g. like
this one (click here)), which I found in a quick search and which i.a. claims to cover "format conversion (SSA <-> ASS)" under Windows 2000 or Windows XP and "which might work on other versions but not tested").