If you're talking about the basic settings in CoreAIDef.xml I've glanced through those (no major test runs with modded values yet though). It seems to control what the AI research, builds, their relations and some tactical combat priorities. I didn't find anything about monster hunting, or how to influence how they go about doing it to "train" their units, and to be honest I don't expect to. From what I can observe from my game experiences, the AI will just gather the closest available force and use those to protect their resources if threatened (or they will just stick it at their border), which makes sense to some extent. I'm almost convinced that it is the monsters that actually attack them instead of the other way around, because most of the time they will just stand around doing nothing. As a result, most of the times, their high level units are mainly junk they had early in the game that managed to survive. They are also notoriously bad at keeping valuable units alive, the most obvious case being their sovereign (see all the sovereign suicide threads).
A player, on the other hand, is the opposite. Not only does monster hunting give a good Gildar return, it is also really easy (and cheap) to gather some newly created troops (say a few lord hammer company) with an imbued hero, attack a high combat rating monster (cast deflect on your LH company and they will mow them down with no losses), gaining 3-5 levels and triple or quadruple your unit HP, making those new LH units many times more powerful than they would be otherwise. At that point, the AI might as well fold because it just can't possibly compete. Thus, until the AI can be taught this behavior and various other leveling strategies for their heroes/sovereign (and it won't be possible through xml), it's really not a good idea to mod anything specific to the leveling system, unless just to make it harder to level in general (and thus harder for players to abuse).
Having said that, if you really wanted to, you can definitely do it with the current modding system. As you mentioned, it is very possible to make an item give increasing amounts based on the wielder's level. Just mod in a new item, or change an old one, use Calculate on the unit's level and multiply it by however many HP you want it to increase by each level. Then just add that item to the unit in question through modding or by adding it to the shop.