Futaba did the "trainer button" deal on the 6XAS. I bought two of them (teacher & student). My instructor occasionally would say, "I have to take it for a few seconds to rest my thumb," then he'd give control back to me. I tried gluing a penny to the button but it really didn't work that well. Eventually we got a 9C to be the master transmitter and it has a momentary toggle switch instead of a button.

I talked to a local radio service center about replacing the trainer button with a trainer switch and they said the way the 6XAS was made it would not be a practical modification.