That would make me very uncomfortable! In my old dojo, we used to do that to lower rangers who thought they knew everything. We made them put on someones black belt and the black belt wore the lower rankers belt. They soon found out that they weren't ready for a black belt and didn't appreciate being treated like a black belt.
Your situation is completely different, but I would still be embarassed and feel uncomfortable. Not to mention, at that dojo, if you forgot your belt for a class, you got demoted to a lower belt for a month. That's what is so great about being a white belt or a black belt. You can't get any lower then white right!
Oh, I was embarassed all right. I had been practicing for 10 years at the time and hadn't "graded" for a number of reasons (changing dojos several times, for one) but the sensei I was asking about borrowing a belt was the shihan for Canadian Aikido Fed. He laughed and handed me the borrowed black belt and said something along the lines of "it's OK, wear it today" when I protested that I was only nikyu and shouldn't be wearing a black belt...