Okay, took a gander in the files and found this that you could change if you were inclined and confident to do so.
File: nUI_UnitPower.lua in nUI/Units folder
Line: 890 or thereabouts
Code:
if frame.bar.enabled then
frame.bar.updateBar( unit_info.pct_power, unit_info.power_color );
end
If you change pct_power to cur_power it looks like it will display the current power ( in your case mana ) value. Not in game at present to test it at the moment but the value should exist and thus display unless it gets overridden at a later date.