Warcraft 3 Frozen Throne has by default a Map called "Skibby TD", under "Scenario" folder I think. There is a race in that TD with towers that have an MP bar.
As lender said, doesn't look like the HP Bar, and requires some calculations and string blah blah, there are some generators of life/mana and casting bars here in the spell sections.
I would not recomend it, because I don't like it. I think it's better to create some kind of special effect like the "!" sign when the unit is low on mana or hp (it can be blue or red), so other players can see what you're needing and refresh your hp/mana if they can, or help, or whatever.