Decorator to convert a function's result into the desired units
Your proposal…


Length.from_velocity_time(100*u.km/u.s, 100*u.h, result_unit=u.AU)



… is less conventional and not any better than

Length.from_velocity_time(100*u.km/u.s, 100*u.h).to(u.AU)


In addition, monkey-patching the from_velocity_time() function to support a special result_unit named parameter feels slimy.

The only drawback with .to(…) is that pure numbers wouldn't support such a method.  But that would mean that the calculation is complete nonsense anyway.