Here is the discussion thread that was about the glyphicons. https://forum.mendix.com/questions/6264/Thoughts-on-Mendix-5--Bootstrap-Bootswatch-collection
It is possible to use the glyphicons classes on buttons, but this isn't currently a option in the properties of the button. You have to add the glyphicons class to the button. The bootswatch module contains an example.
I have been discussions with RND about this feature and I know it is on their roadmap to implement.