I just tested this out, but unfortunately it doesn't seem to be working.
Here's the registry change:

I made the above change, then rebooted my device.
My rules are set as follows:
Rule 1Rule type: Incoming Call
Filter type: Specific Number
Phone #: 1234567890
Enabled: Always
Action1: Pass on to next rule
Action2: Launch Application
App Path: \windows\SendDTMF.exe
Param: ,,,c,,3,,h
Rule 2This rule simply sends an SMS.
I tested this twice with me on a phone call. The phone call I was on gets put on hold and the incoming phone call is answered (I don't believe the DTMF is actually sent, though).
Any thoughts?