Hi, this is my first(somehow useful find).
Today i queued for random dungeon, when it called me i entered the dungeon and cleared all the way to the first boss with my group.
We engaged the boss and after some seconds i got killed(the boss was at 80%), i then left the group when i was still dead(with the release spirit window popped) and then released the spirit and entered the istance to recover my corpse.
I obviously wasnt in the group and needed to teleport myself to orgrimmar(i'm a mage, and couldnt use the teleport out feature cause i wasnt in the group) after a couple of minutes that i was in the city the text "You receive currency x70 Justice Points" popped meaning my old group killed the boss without me.
Step by Step
1. enter a dungeon with dungeon tool
2. engage first boss, die and leave the group.
3. enter the istance to get your corpse
4. leave the istance and hope ur group will kill the boss and earn you some justice points
I dont think u need to die in order to make things work but since this is what happened to me i'll write it, if me or some1 else will try it without dying i'll edit step 2 and 3.
Here's a screenshot.
Conclusions
Now, in order to get justice points it's faster to just do the whole istance, but in this way u can just afk for some time, kill some trash slacking around and then queue again with very little effort and you wont get saved.