Searched for LeaveParty() and just found the usual Ghetto Hearth methods.
This one is similar but should work in raid instances and heroics.
If you're the last person in the group after everyone else hearths out, simply type
/script LeaveParty();
No need to promote anyone. This is easy to test, just be in a group when you enter an instance, still as leader leave the group and wait to make sure the timer doesn't pop up. Then simply type the above and the timer should pop up as though you promoted someone.
Friend and I used to do this when we were duoing heroics as druids and only one of us could get the hearth through the usual method of promoting.
As I said this is useful in raid instances (I know it works in Onyxia's Lair for sure, haven't been in Naxx to test there though) which normally won't port you once you're saved.
Enjoy.
Edit: Yes I realize this is some old stuff here, but didn't see any other threads mention that this will work with raids/heroics that you're saved to, just the usual RFC/Stockades business.