Running a non-rechargeable battery in parallel with an external power supply, regardless of the voltage, will cause your battery to leak/explode. Do not ever do this! Your tech mate is 100% correct in this regard.

You don't have to concede defeat, just use the right jack dude. You don't need to be reinventing the wheel here.

Also regarding current draw, a notable exception are pedals that use a built in voltage multiplier. (MAX1044 or similar). I have a PT80 Delay that steps up 9V to 12V and it'll suck a 9V down in minutes. Older digital multiFX are shocking too, I have a Zoom505 that'll trash a 9V in 15 minutes or so.