Simon - I tried this (it was my first approach).
Originally, I just ran a jumper from the main power feed to the little solenoid terminal.
It was very quick and easy.
Unfortunately, I found about 50% of the time, my starter would "run on", and keep spinning / remain engaged for 1-2 seconds after the key was released.
"forestg" posted up the solution, which is the same as what is provided (upon request) with the DB starter...
The main feed goes to the INPUT of the bulkhead mounted solenoid (direct to the battery).
The solenoid on the PMGR starter goes to the OUTPUT of the bulkhead mounted solenoid.
Works great, lasts a long time (I hope).