To be honest, you don't really need any. You can make up a campaign to run involving the various countries and set up some diplomatic discussions. Naturally, invasions and the like will start and alliances will be formed and broken. Suddenly, you have a campaign. All you need to do is control it. Feed, or misfeed, information to the various factions. Players will start to do some of this for themselves. You could operate a points system for "territories" if you want, a bit like the old GW campaigns, or you could just allow the Victorious army to push back his enemy until he controls the area. The only thing you need to worry about is supplies and communications delays. These sort of games will eventually lead to unbalanced forces, but so what? That happened. Your players may even start drawing a stronger enemy further away from "Home" territory and start harrassing his supply lines and eventually face him in battle with a superior force both in numbers (allies ganging up) and morale. Units can gain battle honours which will give them certain kudos, but ultimately give them some of the special rules from the "Useful Rules" section of the BP Rulebook. Running a blog alongside it is useful because you can give all the players regular updates and 'big up' some of the aforementioned feats. Those feats can be negative as well as positive to add spice. You can send messages to players via email so that nobody else sees or hears them. You can include spy reports and fifth columnist activities. No rules required, just a bit of imagination.
Alternatively you can use the old GW territorial system or other such campaign rules and ideas that are available on the market. Some may even be available for free with a little research. You could even download the Warmaster campaign system and modify it slightly.
The thing is be prepared to do some work and keep it interesting. Intrigue and mystery really do keep that going. The allied players will start to plot and will pass things onto you, as the umpire, to implement. This way you can also make sure that they tell you exactly what it is they want to do. If they leave it to you to assume and you get it wrong, it is their own fault. Works very well.
Another alternative is to use the Risk or Diplomacy boardgames for your map. set up a game and when battle occurs the players play it out using their armies in a Black Powder game. You may get some different outcomes to that of the Boardgame, but it is still a basis for a campaign.
I do note however that there is a photo of siege works on page 33 of the Black Powder rules, but no mention of rules for such engineering, or for that matter bridge demolition, pontoon bridging or any logistic supply, all of which could be useful in a campaign game and might well make a useful supplement, or section in a supplement.
I am probably biased since I spent 30 years in the Royal Engineers and my Napoleonic wargame armies do have all of the components for such engineering and logistic sidelines, which could make a few different scenarios, rather than the standard battle.