If you don't want a front disc, I'd suggest just getting a second fork. Just have the builder make it with a traditional crown, and he'll keep the axle-to-crown distance equal between both forks so the geometry won't change.

Ordinarily a behind-crown brake (oval's A700 comes to mind) would pop up, but it's something you'll want to inform the builder of— most of them require a little more room between the rear of the fork crown and the downtube than most steep & tight track bikes have.

Personally I think having a strange and obviously custom fork is an excellent opportunity to run a beefy mechanical disc up front… if not also tabs in the rear with internal cable routing, in case you ever wanted to use it as SS.