power source for backup battery switch

Hello,   I have a question about the where power it taken from for the backup battery switch,  Rotax Install manual and several other sources I can find have the power coming from the aircraft side  and not the battery side of the master solenoid. In that configuration the master switch must be on, energizing the solenoid for the back up power to work. 

The problem I see is that if I have an electrical issue and smell smoke/hot wires I can not turn off the master and still be able to restart the engine.

If I were to take power from the battery side (or directly from the battery pos)  I would be able to shut off the master to kill power to the main buss but have the ability to restart or keep the engine running with the backup battery switched on( I do have a back up battery for my Dynon HDX).

other than the fact that the wires to the breaker and backup switch would be always energized is there something wrong with this concept? 

Safety wise, I would like to be able to quickly cut power in case of hot electric smell (as is recommended in most GA aircraft with mags) and proceed from there to troubleshoot.

Any thoughts and feedback would be much appreciated.

Scott