Well, to my surprise, these seem to work fine with much longer USB cables.

I just set a rig up with a standard one 1m cable, one 2m cable and one 3m cable. All cards are up and running, no HW errors, no extra or unusual rejects. I really didn't think it would work, but it does!  I doubt I'll ever be selling longer cables, but if someone wanted to run extreme length rigs, it might be worth a try for you. Just make sure to order cables from a place you can return them if it doesn't work, and try a few at a time to start. Maybe I'll put a rack together with 120 cards per wire shelving unit instead of just 60!!