This is the order of operations leading to "Regen agro":

-Regen is on tank
-Tank pulls a pack of 3 with Shield Lob (or just runs into them for initial aggro)
--The tank is now on the hate list of all 3 mobs at (say) 0 enmity, plus whatever was done to the one Lob'd target
--Your Regen ticks; the 2 remaining mobs (or all 3) now have highest enmity built on you

This is unaffected by range. I've gotten Regen aggro (and DoT aggro even) from entirely different floors of a dungeon.

There are next to no situations where pre-Regen/Medi2 is a valid tactic. In some exceptional cases SCHs can have Eos act as off-tank this way but that's about the most outlandish way you can get any benefit from this.