The biggest issue with the mana bars has never been how to make it show (thats the easy part), it is what it should look like.
What happens if my grid squares are horizontal instead of vertical and the health goes RTL? Does it switch to horizontal also and move to the top or bottom?
Width of mana bar configurable? Side configurable? Color configurable? Only show for classes with mana? etc et al.
Personally the low mana indicator has really always been enough for me (and visually it fits better with the overall grid theme and is more attention getting), but I might use something like this if it is suitably implemented.