Flaperon aileron trim when using non-default channels?

I’m a bit confused by something. In the flaperon description at Flaperon Configuration — Plane documentation it says:

SERVO1_TRIM acts as normal aileron trim

How does this work if SERVO1_FUNCTION is not “4” for aileron? Is this really fixed to output1, or could I set a SERVOn_FUNCTION value to “4” for another output channel so I can use its trim for aileron trim?

On my flight controller, output1 is convenient to use as the throttle output. (I can use others, but output1 would be convenient.)