Peter, don't know if this will work, but you could give it a try.
Set up a new mixer, say throttle+, with two inputs, a switch and stick, use the same switch to change between the two control inputs, assign this mixer to the throttle channel.
Make the switch a sequencer, so you can set the delay and slow
This way when you operate the switch it will change to the switch input and start the sequencer, moving the switch the other way will make the stick the input.
Note I haven't tried it and would test this with a servo first, or at least with the prop off.
