I’ve seen other people ask this question - about this and more in general about this for the Star Wars attractions - I don’t know but hopefully they have some plan. Can’t just say, “ok, go out and then come back in the FP line” as people would abuse that.
Might just be a “too bad” scenario as not really sure how it would be handled
Could they link you to the party before/behind you (i.e. use them as the party you need to wait for at the end of the FP line)? I know that I'd be willing to be the "marker" if someone needed it.
And it sucks that when Disney is doing what they can to make the babysitting job easier, people will still try to cheat the system and/or complain about how Disney is trying to help. 