I'd be okay with heads-up runs and scales at Combo races. I know fuel check would be far more difficult to do on such a small level (equipment, knowledge, resources, time), and I really don't think someone's got a drum of 10% Nitro sitting in the trailer just for a heads-up run at a little Combo race.
I think people pretty well police themselves on that level.
While I bracket race my Volare and Turismo, I very much enjoy running S/SS races for something different. While bracket racing the Turismo, I deep stage and use a stripe taker. At Fayetteville's S/SS Combo on Saturday, I'll be shallow, the stripe taker comes off, and it will be dialed welllll below the index!
(Thank God for 1/4-mile racing, in this instance!)