I like the idea of permanent items but I don't think they should be as useful as described here, otherwise what happens when a dude gets say a Mamluk? Being able to use something so expensive for every life for free is just stupid and unbalanced, not to mention what happens 2-3months into strat when everyone has items and a new player wants to start out, it would make it even harder for them to get on battle rosters than it already is now since they have no permanent gear and they can't get on the roster so they can't get the gear they need to be on the roster ect.
I have few ideas about how it could be added but not made unbalanced-
1. When you lead an army, after a battle has been declared you can choose to put a certain amount of gold in "merc payment" and this will be shown on the battle tab and stuff, and what it does is whenever someone during the battle uses their permanent equipment instead of gear you bought for them it subtracts it from there at the cost of what the gear would be like to buy with no discounts.
I think this would be pretty cool for smaller factions who don't have a good selection of gear so they could ask for mercs to bring their own, and of course you could just put 0gold into merc payment and everyone would have to use the items you bought for them.
2. Group items by weight class when buying, so for example you could have "peasant armour, archery armour, light armour, medium armour, heavy armour, plate armour"
And say you bought a +3 Cavalry Robe from your fief and it is classed as "light armour", during a battle anybody could use the cavalry robe like normal, but if they had a +3 Lamellar Vest they could use that instead and it would count as using the cav robe in everything but looks (i.e it would use same stats so people wouldn't just min max buy cheapest armour in each type then try get the heaviest in each type for themself)
3. Your ticket count x50(or any number really) is how much gold each merc has to use on their own gear, for example you have 1000troops = 50k gold, if someone has a Heavy Kuyak that means they can use it 6times before they don't have enough gold left to use it anymore, someone with transitional would be able to use it twice ect.
My ideas are probably pretty bad but I mainly just wanted to post to say how I think it needs some limits rather than just "get +3 armoured horse and plate, proceed to get 10:1 kdrs every battle" but i'm sure a good solution can be found since it's a cool idea.