being able to 'demilitarise' your borders, which would save those florins
I do this where possible anyway. It's especially easy with gunpowder. Just stick an RK and a volley gun in a Citadel/Fortress, they can't be starved out and most armies are torn to tatters before they reach the inner keep, and if they do, you've a unit of FootKnights with a moral boost since they can't run away! If you use ex-heirs you can get 4-5 valour FootKnights right from the go.

I hate it when allies help you out, usually they do nothing but get cut to ribbons anyway, even when they've overwhelming numbers.

If you want to sting the HRE, pick your targets carefully. A few times as a set upon French King, I've managed to spark civil wars by claiming several small victories in a single turn.