The same thought came to my mind as well, could you use another head on the combo?
The speaker output jack will be connected parallel to the amp and built in speaker, I would guess.
No idea if there are any electronics in between nor if adding power into the output jack could cause harm
Another thought could be to add a switch which can decouple the amp and the built in speaker.
If would require a bit DIY but would solve the potential problem of power going back to the Fender amp.
No intention of trying it out with my nice 3 Rumbles though, I'll leave it up to the experts

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