Y'know, there's an "out" clause in the Montpelier series rules (hasn't been needed so far) that may be something to consider for pavement; "At the race director’s discretion, class may be split into two classes (Midgets and Mini-Sprints) if car count or competition level necessitates it."
It could even be a race-time decision, based on how the cars that sign in perform in hotlaps; if any TQ's, Kenyon Cars, Rookies, Old Guys, etc., seem to be an impending hazard, run them in separate heats and features.
Better than cluttering up the rulebook, in my opinion...