Got this working now. I had to reflash my Slug with Unslung 6.10 firmware and then run ipkg update to get the new optware packages.
Once that was done I ran something like smbmount //192.168.1.1/Music /mnt/music -o username=USERNAME,password=PASSWORD
That did the trick.
Unfortunately I've now got another problem in that despite adding /mnt/music to the list of drives that Twonky should index, it doesn't seem to be doing that.