I was looking into it a few months back but I'm holding off for the moment.

I made the same assumption as you, can't exactly remember why but I got the strong impression those events (at least rally sprint) would be sufficient.

You have to present the car once a year for inspection by a recognised CAMS scrutineer (not DoT), but I wasn't sure if scrutineering at certain events would cover this or not. I think at the time of the inspection they check your log book to see the usage of the car. The DRC program has specially "Rally XXX" plates for the car as well, but I heard DoT have no idea what is going on and don't issue them most of the time.

All in all, I started to get the idea that the best thing to do if you want to build one yourself would be get a road registered car and get it kitted out (cage etc.) and just keep it on a standard rego as long as possible. With minimal road usage you'd potentially get away with it for a while and then just bite the bullet once it gets yellowed. It would be the same process from that point (from what I read). Buying an unlicenced car would be pretty frustrating when getting it from engineer to workshop etc. for all the work required. Permits to move only cover to and from inspections