The trial/time attack idea is pretty cool. I have been an OTD participant for a few years now and, at times, have thought about competing in some way. My main concerns have always been..
1. contact. I won't risk racing door to door racing in my 50/50 street/track car. That's just not on the table for me. I know that OTDs are inherently risky based on the activity itself.. but if I wreck I'd prefer that it be MY fault.
2. expense of building the car to meet certain club guidelines - scca, nasa. Since I only participate in OTD, HPDE, I like the freedom of doing whatever I want to my car.
3. ties to #2 above slightly... tires. I do not have the resources to trailer my car, tires, equipment. I prefer to stay away from the 'black-crack' and keep my street tires and ability to drive it as is to and from the track.
So, it looks like you have considered these items in one way or another. Based on the replies so far I think my only other concern would be actual track time. I really enjoy getting a full day's worth of track time. At least at the beginning of the season, I don't think I would sacrifice much of my day to participate in a time-trial event - when I could get 60+ minutes of OTD time. My thoughts on this might change after that first OTD of the year, initial rush is cleared.
I understand that everyone is different - I am just providing some other feedback and a potential participant's point of view. Thanks for the discussion - and thanks to all the staff and volunteers for years of fun out at Waterford.