Without hooks into they way windows draws things, not really, and the order that xojo draws controls right now, possibly never.
Here’s a really, and I mean REALLY crude example of getting it to work with two canvases over the top of each other that I just whipped up. There’s a lot of work in there to make it more generic (e.g. a generic checker for controls currently under it instead of hard coding one control in the open) but as a proof of concept its doable, but alas, not with controls that don’t have a paint event.
Honestly though, you’d be better off doing as suggested above and writing your own button code for a canvas, you won’t be able to venture outside the canvas’s constraints while dragging but it will be the most crossplatform.
Another method might be to use local drag and drop where you can set the drag image to the “button” so it looks like you’re dragging the button around, but I’ve not really had a play around with that so it might not be doable.