The easiest way will be to look at oQueue and join a group that's hitting the old bosses for the chance at the mounts.
Barring that, you might still be able to fade the aggro from him onto the guards next to the raid portal (might need dispersion to make it to them, I haven't gotten that far on my priest alt, so this is speculation).
any spell that drops threat (vanish / Feign death) etc will let you aggro the boss, toss the spear, and then "lose" the boss after the add has spawned and started chasing you.
Not sure if priests have any spells that effectively dump all threat (shadow might have something, not sure).
As a priest, your best option might be to spec into anything that gives you a movement speed boost (angelic feather thing? / body and soul), and try to aggro the boss, toss the spear, get the add on you, then pop dispersion + speed boost and book it far enough away that the boss resets. I have heard it is possible to get the guards near the ToT raid portal to aggro the boss, which might be an option if you can survive long enough to get the boss stuck on the guards and kite the add away.
of course, easiest way is just to probably put together a quick pug for world bosses, and knock it out while the raid rapes his face.
Pull, use Spectal Guise, kite add while boss resets.
Don't run towards ToT as the guards over there are likely to attack you if you do.
(They are only there to keep people from fending off gankers anyway.)
I did this last night, the quest item has a really large range (further than your spells!), I stood really close to the ToT entrance, used the spear, and just kinda backed up into that room. The guards held Nalak for a few seconds and the Spirit thing just slowly slowly moved towards me, losing tons of HP, by the time it was even within range of my spells it died.
I didn't cast a single spell, just the spear to pull Nalak and that's it. No need to overcomplicate things.