Hi MylesRip,
There is no way to access the ScreenTip programmatically, either through code or the designer. This is annoying and last week at the MVP Global Summit I let several members of the InfoPath team at Microsoft know that they should really expose these things as part of the object model.
Lastly, I really doubt that ScreenTips will work in browser-enabled forms since it would take quite a bit of AJAX to do that, and browser forms are just not that developed.
Matt Faus / Microsoft InfoPath MVPQdabra® Software / Streamline data gathering to turn process into knowledge