... and all that hard work was seemingly pointless (but not without a happy ending!)
The latest Infocast 8 firmware does seem to include CIFS support (and I presume this applies to Chumby 8 as well), so the Chumby can natively access your Windows shares. It looks like someone has also made a Chumby app for that, which you can find here -- he's posted full instructions on how to use it here.
For those looking to mount Windows shares for other purposes (hacking scripts, etc), the commands you're looking for are:
mkdir /mnt/winshare
mount -t cifs //ww.xx.yy.zz/myshare /mnt/winshare -o username='myuser',password='mypass'
1. Replace /mnt/winshare with whatever Chumby-local path you choose (generally though it's a good idea to keep it under /mnt)
2. Replace ww.xx.yy.zz with your Windows box's IP address (not hostname, or you'll likely just get a "bad address" error) and myshare with the share name.
3. Replace myuser and mypass with an appropriate name and password for the Windows box you're accessing
3a. If you need to specify the domain for the username, use the format myuser@mydomain