I believe that font icons will be supported in a later version of Mendix. However if you want to do this in the meantime you can add the font awesome classes to buttons. The only problem with this is that it will change the font of the text currently. So to get around this you can create some custom css using the font icons css like so:
font: normal normal normal 14px/1 FontAwesome;
All i have done is changed the usual font awesome code to fit better with the current Mendix 5.15 HTML.
As i said earlier this may well change in the next versions of Mendix.
You can now use the Glyphicons in MBM, thanks a lot guys! This really helps.
Really looking forward to custom glyphset selection, really pimp my app out!!!