In the end I stuck it face down on a piece of glass and measured the height of the rim, then turned it over and did it again. I had to compensate for the axle end protruding from the locknut more on one side than the other (and odd bit of Schmidt inconsistency), but it is centred.
It's very hard to get even spoke tension on a small wheel - there's very little elasticity in those tiny p/g spokes, and 1/8 of a turn makes an appreciable difference to tension.