"... altitude where an aircraft is still usefully maneuvrable."
Non-qualified armchair blathering here...
Wouldn't it hinge on the number of Gs that could be held? You're right: the era would make a big difference - An SE5 that could still pull 2gs would perhaps be a competitor, while a Spitifre would most likely be dead meat? It just depends on what the other fellow is capable of.
Stall speed / power availability (for replacing lost energy) would be the keys, yes? (no?...) Reliable data for the aircraft of the time may be difficult (if even possible?) to obtain, but relative best-guesses would suffice for a game (which is what you're after, right?). Start with sea-level, and increase stall with altitude (there's a good discussion somewhere in the forum about altitude/engine performance) and multiply for the G-rating of the turn. The bird that can't hold a turn goes down, leaving him open to attack from above by the other guy who could manage to stay up without trading altitude for energy. Gentle-stalling aircraft (like the Fokker D-VII...and perhaps some two-seaters? dunno...) would have a distinct advantage by allowing the pilot to push closer the edge of the envelope and eek out that little bit extra of performance.
Dunno if that's of any help...just rambling.
