> Five meters can be entered as '5 meters', and 5
> kilogram meters per second squared could be entered as the string of
> messages '5 kilogram meters per second squared'.

We will ignore the problem of plurals (i.e. '1 foot'  and '2 feet').
Its pretty trivial.

How in the world are you goind to define #per  ????

Also, #squared would square the whole result, not just the last unit.

Stick with the (rather nice) direction you have already taken.  Consider the 
rest to be a UI problem.

-- Mike Klein