I like the idea too, has happened often enough I continued editing with the wrong brush.
While it wouldn’t always help me to undo stuff when I forgot to swap back the brush, having hundreds of “Freehand Brush Stroke” entries certainly doesn’t help much.
I wonder how hard it would be to offer more info (in a tooltip or so), like the layer, the bounds of the stroke and such, that information must already be in the undo stack in some form. I think I’ll put that on my list to do when I get bored…