Queue for a 90 dungeon with at least one other person - for the sake of simplicity let's say specifically one other person. Meaning form a group of 2 prior to queueing.
Complete the dungeon. Let the 3 randoms leave so you and your buddy still have the opportunity to use the teleport out of dungeon feature. One of you use that feature to go back to wherever you were before.
The person that teleported out goes and enters a low level dungeon.
The person remaining in the original 90 dungeon would then use the teleport out of dungeon feature. Then use the teleport to dungeon feature and should end up in the low level dungeon with some abilities scaled.