That's a good idea, it mimics the reality that resource constraints would have on crude warfare. Wars are, after all, not eternal affairs, and in a technologically limited setting where communication, industry and logistics are severely impaired, they would tend to fizzle out before a conclusive victory could be attained. In our industrialised world, amongst nations that can manage and commit resources with utmost efficiency, wars generally last about five years and can easily result in total victory; this is the kind of war that BM experiences now. Going back through time, to the age in which BM is set, you begin to find wars with excessively long durations. The Hundred Years War, the Thirty Years War, the Byzantine-Sassanid War of 602-628. These were wars that could last generations. Of course, they weren't singular wars, but rather sporadic bouts of fighting broken up by periods of peace, and that is key. The states of the world back then were so base that wars could drag on and on due to their limited capacity to wage them. This is what the type of situation that resource constraints could lead to. Conquests limited to whatever can be achieved within a controlled amount of time, via resource factors. Sounds good.
You might also want to take a page out of Machiavelli's book, The Prince. In it he mentions the difficulties states experience when trying to assert themselves in unfamiliar lands. The locals do not care for their new, foreign masters and so the new lands require expensive garrisons. These garrisons prove to be costly, perhaps resulting in a net loss, and are frail, in that they can be easily disrupted. So, relate these things to BM somehow. The ways are obvious and already done to some extent, but they can be made more extensive. Intensify the troubles that realms have in holding conquered lands far away from their capitals.
Also, Scarlett was always talking about how scorched earth style looting is unrealistic. Perhaps looting could be changed as well, so that it can only harm a region, not turn it into uninhabited wasteland.