Well, try it for yourself : smoke your dose of changa in a normal pipe
I do believe though that the leaf material used has an influence on the trip indeed, even without enough harmalas.
You might be able to vaporize harmalas in the GVG by getting your torch closer to the stone, but it will still be much less efficient than with direct flame contact.
Also note the harmalas have to be kept for longer in the lungs than DMT to pass in the blood stream, so it's better to hold your breath up to 30 seconds to absorb most harmalas... Or, like me, use a massive dose of harmaline in your changa and smoke it by direct combustion instead of vaporization to compensate for these 2 "issues".
When I used to vaporize my changa, the effects were much closer to a DMT trip than a small Ayahuasca journey like they are now for me (joints with 150mg 1:1:.5).
To me, changa is not about prolonging a DMT travel, it's about transforming the experience completely and making it more shamanic so I can use it in ceremonies...
To me, changa is really about the synergy between the Power (harmalas) and the Light (DMT)...
Changa is Ayahuasca.
To each his own I guess.