This is troublesome, because if you forfeit the match then the pet will despawn and you lose your shot at it. If you force quit WoW and relog, the pet will not despawn, but it will instead revert to being a clickable critter, and you can interact with it to start the battle over again. The pet will retain all its former characteristics (e.g. rarity and stats, and the secondary pets that join). Of course, you risk somebody else coming along and grabbing your pet spawn while you reload.
This is obviously not a very elegant solution, but so far it's the best way I've found to avoid losing the spawn altogether.
*edit* console reloadui doesn't work, actually, but forced relogging does.