I've asked about this a couple of times, and only ever got an unhelpful "truing doesn't really matter on a trike" as an answer. Which is fair enough, unless you're of the OCD persuasion, or want to build or repair a wheel or something.
Anyway, I finally discovered a proper sensible solution, in the form of something called a Thru-Axle hub (some kind of mountain bike thing, m'lud) adaptor. These can be
obtained cheaply on ebay, and the 12mm version is a perfect fit for the ICE front hubs. Your trike/trailer/wheelchair hubs may vary.
They simply insert into either side of the hub...
...to give a conventional QR sized hole.
A skewer can then be inserted and the wheel fits in the truing stand in the usual manner:
(I've bunged a few nuts on the skewer to adjust the length for the width of the hub.)
Simples.