This siege topic is sadly without solution (only if CA makes it).
I was thinking about that setting "legion on leave" for maintainance purposes. The answer was right. Its just too much scripting...but...
If there is like new army stance "on leave" then problem is solved...none unit looses exp because it wouldnt be disbanded at all. Main difference is, it still uses "number of armies" count but still. Point was money.
Whole idea:
new stance "on leave".
Maintainance lowered to fixed 10% (cant get lower)
still needs support train/near town ect as any army
Can not move and is always ambushed if attacked.
And mainly. Attrition rises to 99% it would imitate disbanding legion. = If you turn off this stance to normal, it would take a few/several turns to gather to full units legion, according to replenish rate, as any army.
Difficult part is, it even drains manpower from region, because of everyturn replenishing...therefore it must add manpower to region to imitate not depleting locals but rather opposite. Or is there option "cant replenish"?lbecause it would solve this manpower part.
At last. Lowers public order and food. Yea. They are profesional killers. What they do on leave. Criminality...
Only issues are, that not real disband = army count and bigger problem. AI. No way it would understand it without real wide scripting. So make it just for players. Minor one is paint a new symbol for stance.
(Fingers crossed for no answer as not possible hehe)