I'm gonna chime in with this:
It may not be a Code violation, but it is just plain dumb in the sense of false economy.
The added cost of the wire (let's say for sake of discussion that the conduit will be sized for full-load conductors from the beginning, which IMHO is a MUST), at the time of original installation WILL be a LOT less that the costs of wire (remember you're gonna be paying for the half-load wire AND the full-load wire) and labor to re-pull the full-sized conductors later.
Do it right the first time and avoid the extra costs and downtime in the future.
Look at it this way, you have already saved a bundle by having the transformer on hand, so even with full-load rated feeders you're still way ahead on the money. :grin: