I've been thinking about a new design for the API, which does not rely on Symbol as the kind of base units, and does not require the quasi-quoter to write literals.

The library should provide this:

data UnitKind

type Unit = UnitKind -> Type

Now user code can define empty datatypes representing base units, and type synonyms for units:

data U_m :: UnitKind -> Type
data U_s :: UnitKind -> Type
data U_kg :: UnitKind -> Type

type U_N = U_kg *: U_m :/ U_s ^: 2

and at the value level:

m :: Num a => Quantity a U_m
m = MkQuantity 1

_N :: Num a => Quantity a U_N
_N = MkQuantity 1

thus we can write:

5 *: _N /: m :: Quantity Int (U_N /: U_m)