The ButtonTool derives from ButtonBase which provides the ICommandSource properties such as CommandParameter so that should work just fine. Perhaps the issue is related to being in a different focusscope, does the same code work if you use the Button class in a toolbar? I would probably recommand submitting a sample that shows the problem to the support group so we can look into the issue.
In the end I coded around the problem (didn't use CommandParameters) but I did try to make a small sample app to reproduce the problem, but as often is the case, it worked!
So short of rolling my code back I don't have any way of reproducing the problem.