The problem with that scenario is that I have no idea how DB25 SCSI works, and I'm not convinced that everything will get terminated correctly.

Boy, I'm not sure there is a good solution here. In a perfect world, I'd have a cable that had three 50-pin connectors on it, hook the scanner up in the middle with an adapter and terminate after the tape drive. But I'm not aware of an external cable that has more than two connectors. I might look at something like the external->internal->external cable like the one that's likely inside your tape drive's case.

This might all be overkill anyway, and it'll work because DB25 will somehow terminate all 50 pins properly. But I just don't know.