Say I make an HS mule...

You can always do what my JP friends were doing with their HS mule. Stick them in the previous map at a safe spot and when you need HS, bring the HS mule back into the map, give HS, and then go back. Even though it takes a few seconds away from grinding, the extra EXP should give more than enough to make up for it.

That's only 110% EXP training, as opposed to the 121% he'd be getting with the HSmule leeching in the same map.