Thanks for the tip Erick, but I ended up buying another Epson 2450 for $50, and a newer 3170 for about the same price. They're fine scanners for MF. Actually, now that I bought these, I wonder if I need them. Just printed up some 6x6 and 6x9 negs at actual size to simulate a contact print, and you can see quite a bit from a print that size. What I can't tell anything from is a negative. Oh, I can see if it's exposed correctly, but I just can't visualize how it would look as a positive. So for proofing, I'll need to go w/ a scan or a contact print.
The Epson 2450's have an oddity about their film holders as well. They don't hold the neg very flat, but several times I've taped the negs flat to the scanner's glass itself and saw no difference in sharpness compared to a rather warped looking neg in the holder. It doesn't seem to matter that the holder keeps the negs a certain distance from the scanner glass either, as my taped-to-the-glass scans looked exactly like the ones made in a film holder.