How about just changing the display? If I type 5ft 3 inches into a constraint, represent it internally in mm but leave the text as "5ft 3 inches".
Display mm on mouse over?
Unfortunately here in Canada we do end up having to mix units a lot, and it seems cleaner to me to make sure everything is displayed as what it actually is. 5 foot three inches might technically be 1600.2 mm, but that doesn't help if I'm converting a technical document from an imperial source.