I think looting needs a button. Either put it next to the assault fort button in siege view, or put it on the army next to forced march. It should work similarly to either.
If on the siege view, it should a tick of attrition, possibly per basetax (so a 5BT province would kill 50 men in a 1k stack if there is no +attrition modifier, but reult in 5 ducats and giving the province "looted"). It should only be available on non-core(maybe) and non-occupied provinces. It could cost at MOST 5 military power, and if it does, it should cost at MOST 1 power per 2 BT.
If on the army view, it would be a toggle, just like forced march, and they would have the opposite effect (possibly even gaining a mil point for each province looted, because you're gaining experience) and slow the army down, while increasing reinforce rate (everyone loves looting). An army can either be forced marching or looting, not both. When a looting army sieges a province, they will wait for the siege tick to pass (all 30 days) then be take attrition, get the money, and give the province the looted buff. If the province already had the looted buff, they won't siege it and won't take attrition. There could be a die roll to succed with fort level possibly being replaced by basetax, cannons being replaced by army size and leader siege value being replaced by cavalry. Oh, and make it so that having an army of equal size to basetax means it would always take one tick.
Making cavalry better at looting makes sense because 1, hordes usually have lots of horses and 2, they can cover ground faster.