Hello Kevyn,
Welcome to the forum.
The Pell register system features auto cancel (auto declanche) which means that the arranger only has to select the register he or she wants without worrying about previously set registers, or including a cancel signal. This is all handled by the hardware on the Harmidi system.
However, if you are using a J. Omega board or similar to control your magnets, there is a register control board available from a couple of sources (including John Smith). It is simply wired into the existing system. It can control all the registers, including the multiplexed tremulant signal. The only limitations are the number of magnet ranks, as they are driven by single driver chips instead of a dedicated driver for each rank. This limits you to about 3 ranks depending on what magnets you use. However the problem is easily overcome if you want lots of ranks.
I hope this helps.
Cheers,
Rob.
http://www.robbarkerorgans.co.uk