You could, obviously, turn off the heater and run the fan, that would still generate the chuff, and I don't see any issue doing that. Probably the cheapest way to accomplish the goal.
You could really have a DP switch and turn off the fan as well. You could still tap in before the switch to extract the chuff signal and ship it to the tender. You can actually do all of this in the primary tender, all the wiring is there.
If you want to get REALLY fancy, you could do a wireless link to the water tender.