I keep popping on the USARC website and having a look around and I'll no doubt shoot some at some point.
I think it's a great idea and hope it gathers even more momentum. It is difficult for you to try and get a minimum set of rules that will cover all instances and keep it basic and attractive to all.
I started shooting HFT when it first started and I remember then most people shot basic off the shelf kit. I remember the local FT guys saying it will become FT2 with the kit race kicking in and before long people would be using top of the range target rifles and scopes. That has happened but it has still kept it's own identity re no sitting and no scope twiddling.
I know you will have discussed all this but one of the problems of trying to make it 'off the shelf standard' is that that in itself will benefit some and penalise others. Shooters aren't standard off the shelf sizes and shapes ... hence the adjustable target styles I suppose. I suppose if you say that the cheek can be so high in respect to the action or the front end can be so deep in respect to the bore then it gives some room for body shapes ... but that gets you back to Gary's original question of what's the difference between an adjustable xxxmm or a solid wooden xxxmm. Sooner you than me.
Good luck with it thou and I hope that the kit race doesn't take it away from what it was intended for.