Dave,
Since the SB software is written in perl I suspect that the directory structure and control files are the same regardless of version.
The file that controls which firmware the SB uses is squeezebox2.version. This is just a text file (UNIX format). It can be edited with any text editor. The directory tree on a PC is Slimserver/server/Firmware and the file lives in that directory.
If you feel comfortable editing the file it should be no problem. Make a copy for the control file before making any changes.

The code below will load the V15 firmware. Other versions are the same except all instances of 15 will be whatever firmware version you are running, such as 28 or whatever.
# Squeezebox2 firmware
#
# from to
# from1..from2 to
1..15 15
# future versions may be downgraded to this one
* 15
If you're running OS-X I would think it has perl built in (since it's UNIX based) and you should be able to run a perl script that would do this easily. 'Course that means some one would have to write one.

On other possibility. Since you mention having a PC, if you can enable PC file sharing on the Mac (Samba?) and can map a drive on the Mac from your PC, you could run the SB firmware app on your PC and have it write the file directly on the Mac.
Good luck,
Mike