Incomplete & untested solution:
Software switch (unverified):
This shortcut will open steam in big picture whether or not steam is already running:
C:\[Path To Steam]\Steam.exe -start steam://open/bigpicture (Source)
your inner Panda - 12 de Sep, 2012 a las 13:20
[Placeholder for script that maps a button to it (presumably using AutoHotKey)]