Hi Jim,
I am afraid this feature is not supported in the current implementation of XPMenus. We have considered this as a feature request (feature# 447) and will try our very best to implement the same as soon as possible. You could track the progress of this feature request at this link below :
Feature Request #447-Add support to turn off the default tooltip feature for BarItems placed on the toolbar.
We appreciate your feedback and thanks for your patience.
Regards,
Ebenezer.P