Thanks to all of you who have responded. I've already talked to Electrodynamics, and their system (and the SmartFly system) will only work with electronic modules, not magneto ignition. Yes, ground loop might be a problem, so I'd probably have to put a diode on one side to stop current flowing back through the open mechanical switch.

Maybe I'm making too big a deal out of this, but I thought that someone would have already solved this problem with magneto engines. This project is costly enough, and I don't want to spend more on the engines to have them converted to electronic ignition. This is really a safety issue, so if for some reason I lose one throttle servo during flight and that engine keeps running full bore, I'd like to be able to kill both engines and land dead stick. Asymmetric thrust on a B-25 wouldn't be much fun at all!

Perhaps the choke servo is the best way, but I'm still open for ideas.

Thanks, Lee