Hi Kobra and Linda,
sorry about the slow response. I've been sick. Actually the issue has resolved itself and I no longer need to make a "hidden" category.
Not to leave you hanging - the item in question was a donation (fund-raising for a small group of which one of my colleagues is the founder) and he thought, if we could combine the fund-raising aspect with the cart I'm setting up anyway, it would be ideal - but we didn't want it to look like we were soliciting funds for something that's really not part of our own line-up.
Anyway, as I said, the issue resolved itself. My colleague has decided he wants his group to have a zencart all their own. They've even offered to pay me to set it up for them - tee hee...