Interesting problem! I was wondering why even define things like SquareMeters, but as you say the issues is reducing the terms that have different forms but mean the same thing.
Having a SquareMeters type was meant to reduce complexity. Instead of always dealing with Area<Meters, Meters> all the time, it was meant to be a shortcut.
Hrm, what about namespace aliasing? "using SquareMeters = Composite<Meters, Meters>"? Since the problem is not the composition of the type but the verbosity of the deeply composited types, then perhaps it's just about making a shortcut for the name.
Okay, I've definitely convinced myself there is no good way to do this without having to write way too much code over and over again. The problem is basically equivalent to inserting items into a sorted list, but you only have one operation in which to do it. So, you are either limited to simple types, or your code for operations on types explodes trying to define all of the ways to compare types (in a generic sense) and different scenarios for combinations of types.
Hrm, what about namespace aliasing? "using SquareMeters = Composite<Meters, Meters>"? Since the problem is not the composition of the type but the verbosity of the deeply composited types, then perhaps it's just about making a shortcut for the name.