In the functions list (at least in Mathcad 11 I don't know about previous versions) there is a function called
UnitsOf() I had to use it for a function that was raised to a power of 2/3. It solves your problem but it makes the equations look bulky because you divide by it and it actually displays as "UnitsOf(X) in your worksheet.
Wish there was a better way.
DPA