
1. Add a button in the buttonbar
2. In the command, set an existing EXE
3. Enter a tooltip containing the & character (i.e. "Test1 & Test2")
4. Press OK to save the buttonbar
5. Rightclick on the button.
At this point, a menu appears which has as first item the tooltip of the button. This item appears as "Test1 _ Test2". Obviously when setting it, an extra & is needed.
I guess it can receive the priority it deserves (infinity -1

P.S.: For some reason, this sounded a little familiar. Something similar has been reported before, and the search revealed this.