I am not really in to HARP, but I do know a thing or two about armor. I was reading the post, but why not, as a sort of optional rule, allow armor stacking. Using Kurbul and mail together actually is a very good idea, since it counters all types of attack better than each on its own.
Kurbul wards off cuts and crushes, while mail counters punctures and cuts.
Perhaps apply a small stacking penalty additionally with all the other penalties to balance the DB boost or have DB stacking limits per combination: so mail and rigid leather would stack 75%, mail and scale just 50% and mail and plate 25% (from the assumption that the better DB bonus is the base and the mail just adds little benefit).
Plate and Cuir bouille do not stack appreciably, as well as soft leather and mail. Leather scale and metal scale shouldn't stack, too, all on account that they essentially work the same way, except in different materials.
Locations for such extra armoring are usually well thought out by the armorers of old. Such as the coif: it was a real problem for knights to get hit on the shoulder because of the large area, availability and its weakness in that it allows upper body maneuverability.
Extra armoring was commonplace and if it can be simulated easily: I'd use it!