Thank you for your replies! They've given me some food for thought.
The object is not a pie plate, but I'm really paranoid about internet anonymity, so I would rather not say exactly what it is. It is an item that has a specific purpose for which I will never use it, and an item that could be considered decorative but is not to my personal taste. A pie plate is a reasonable stand in. I know that without saying what it is, I'm not going to be able to benefit from people's ideas of how to repurpose it, and I'm okay with that. (As much as I appreciate your impulses to solve the problem for me! You guys are all so nice.)
I'd really hesitate to give it back to her, or even to donate it locally on the tiny chance that she'd come across the item in a Goodwill store or something. Have you moved during the years that you've had the 'pie plate', or might you move anytime soon? It may be virtually unbreakable, but pretty much anything can be "lost during the move"!
I have moved, and pretty recently. That's not a bad idea if I decide to quietly get rid of it.
Does this thing really take up that much room to store? Is it not possible to pull it out and use it when MIL visits and store it away under the sink or something the rest of the time?
I would be hurt in MIL's place, even if I didn't want to be/knew I shouldn't be.
It doesn't take that much space, but my house is pretty cluttered and I'm trying to clear things out. I'm actually getting rid of things that have more sentimental value to me personally but that I no longer use (ex. figurines that I had in my childhood bedroom that just aren't suitable for "grown up" decor), and I'm keeping a number of other things that MIL has given that I just don't like. She has a knack of giving decorative gifts (framed photos, lamps) that are not my style at all, and we do display/use several of them in the name of not hurting her feelings. Maybe it's silly, but I'm drawing the line at this pie plate. It may end up with MIL, it may end up in goodwill, but it's not going to be here any longer!