This is a quick work around, and not the best, but it works until XSplit adds the option to bind anything to PTT.
The method I used was to rebind my caps-lock button so it instead presses a button that works for XSplit. I changed caps-lock so it presses F8 and bound that as PTT in XSplit.
You can rebind any key with a program called SharpKeys that you can download here: http://www.randyrants.com/2011/12/sharpkeys_35.html
Obviously this makes my caps-lock not work as a caps-lock, but personally I never use it for that reason anyway.
Hope this is to any help for anyone, and sorry for the short "guide".