As for replacing a hall effect PCB with a reed switch or other type of non-powered switch, its easy.
You may want to chase this down with a multi meter, but I believe pin 21 on the RS 4.0 audio board is the chuff input (where one leg of the reed switch connects to) and the other leg connects to AC grounds (pins 3 & 4) on the same audio board. Be sure to remove the hall effect sensor first! (going from memory here! sorry I do not have a sound board handy to test this.) This does not work the same on an RS3.0 audio board!
The other place you could connect the reed switch, for a steam loco would be on the R2LC, specifically pin 17 and AC ground. Pin 17 takes the ground pulses for chuff in and encodes the output (chuff trigger) into the serial data which is then transmitted through the IR tether and decoded in the tender. This way the reed switch stays in the loco where the drivers are and no wires between loco and tender!
For locos with RS2.0 platforms, just chase down the cherry switch, one pin is ground the other goes to the LCRU (to be converted into serial data). So, same results, reed switch is in loco, railsounds in tender and wireless IR tether in between the two!
Thanks,
Mike
