On touch screen devices, we currently allow the user to change between mouse button 1, 2, and 3 by pressing an icon. A more "standard" solution seems to be to press and hold. For example, from http://windows.microsoft.com/en-us/windows7/using-touch-gestures:
"Press and hold does the same thing as right-clicking an item. "
However, we need to consider if/how button 2 should be supported.
If looking into this in the future, we should consider adding support for other touch gestures as well. Some ideas are loosely mentioned here: https://groups.google.com/forum/#!topic/novnc/2rdr_ZxP1rA
How Microsoft does gestures in their Android RDP client:
This got fixed as part of bug 7111.
Tested and works fine with gestures.