Yes that's a snubber, but it attaches to a QR skewer, not a derailer hanger. I suspect the ones that attach to the hub axle or skewer will be annoying to me, because they come off and flop around every time you take the wheel out, and need to be re-positioned each time you put the wheel in

The kind that attaches to the hanger will stay in place through wheel changes. Plus the hanger looks less dumb sitting there useless, when you put it to use for a snubber. So I think of the hanger-mount kind as more elegant, though both kinds will probably work the same on the road.

That one is a tensioner. I have no use for one of those. You did say modified, but I can't see any way to do that, that isn't more work than just making one from scratch.
It can't be spring loaded, it has to be rigidly attached and immovable enough to prevent the belt from riding up on the teeth of the sprocket when overloaded or under-tensioned. So it needs to be keyed to the hanger with no ability to rotate. And a toothed chain pulley is not a good shape for pushing against a flat belt. So a couple pieces of that might be usable as just metal chunks to make my snubber from, but I'd rather start with a blank slate and make it the way I want, rather than be constrained by the shape of that $23 part. Plus I have all the raw materials I need, so mine will be pretty much free, except for my time. As a hobbyist, I don't count my time as worth very much.

Thanks for thinking outside the box though.

Mark