Not sure if this has changed in the last year or so but in my experience it is, at the very least, up to CM discretion to allow a tall enough 'non-rider' to do the rider swap. For us, this is how it worked:
Dh and I and one kid wanted to ride TOT, ds7-ish at the time, did not. So we all went in the line together per the CM instructions we were given at the entrance when we explained one child not wanting to ride (we happened to have FP's so we went in FP line). When at the very top...CM told me to let CM know I had a non-rider - so me and ds went down the 'chicken exit'. Once at the bottom, CM instructed us to stay there and wait for rest of party. Once they came down, dd and I stayed near that elevator entrance and when a CM came out - we indicated that we did rider swap --they brought us back UP the chicken exit and put us on the next elevator.
Two observations from my experience:
1) It was not a standard thing. There was no CM down at the bottom waiting to assist us back up - I had to track someone down and wait several minutes to even find someone.
2) I'm not sure if I would have done it had I known - I would have just used FP another time with my dd (of course, with the new enforcements, likely would have to have someone 'approve' late use of it if I have to wait for dh and dd to go through once - depending how long FP line is). DS was very apprehensive waiting in line - reminded me several times 'I don't want to ride this, remember mom?' - THe child in question has to go through the 'scary' movie room and then...ends up on an elevator. He had gone on before and knew that the elevator 'breaks' so he was a bit freaked out about having to get on an elevator at the end of it anyway.
YMMV.