Im doing the exact thing you are right now. Pally-mage combo JUST dinged 30 last night, And ive had that mage shanked from behind so many times.

A useful macro that i spam constantly on the Palidan:
/target {slavename}
/cast Righteous Defense
/targetlasttarget

This gets any mob attacking the mage attacking the palidan instead!

But in terms of the LoS issue. Best thing ive found out to do is strafe around the mob a quarter of a turn, this will cause the follow to reingage and have him face you.
Or even simpler if you are in this situation:
{Palidan}
{Mage}
{Mob}

Just run straight through the mob, your mage will turn around then you can proceed giving divine justice to the mob that was stabbing your mage in its back

Hope that helped!