I like to think that minor clan troops are themselves custom troops.
Seriously though, if such a feature is implemented, it needs to be balanced right. Like if the tier of troops and equipment you can afford is affected by your clan tier and ****.
the best implementation of something like this I've seen in Warband was in PoP mod.
You should be careful with your notions of balance, though, walling things behind grind isn't balance and can rather be harmful to the experience like we've seen so far with the nerf rage TW does, so many things in the game over the past 2 years were rendered either useless, boring or inefficient (along with removal of essential features like the ability to create custom formations by troop, we now got this awful slider that completely destroys nearly all manual strategy inputs)
Instead of locking it behind renown gain - which 99% of the game's features already do, the balancing should be made over income. So, if a troop has the best gear in game it must also have the highest daily wage + highest upgrade cost. Than I'd place the unlock of gear availability over territory & PC culture. If you are Battanian you can have all battanian gear available from the get go, while the best stuff from empire remains locked until you get territory that owns the relative gear... - on a side note, if something should be nerfed it'd need to be the negative daily wages, those shouldn't even exist, or at bare minimum troop wages should be significantly higher to the point that even with 400 steward they'd still cost us something.
So let's say someone wants Imperial Charger horses for their custom troops - your faction must own villages that produce horses and are Imp culture - Want some nice cataphract lances? Phine, must own iron mines within Imp culture territory - so on so forth. Since all cultures hold iron and horse villages that wouldn't be an issue at all and would place a rather alluring reason to capture certain territories. This I'd call balance, not some arbitrary number to encourage even stronger renown rush playstyles.
I said once that, in general, the greatest design flaw with BL's a lack of cohersion and excessive "disjointness" - basically all features are disjointed working independently being regulated by arbitrary number games - instead they should deepen the entire experience into a logical & intuitive system where things actually matter in conjunction. So, owning a smithy with a insane amount of iron mines (Epicrotea for instance) or the same with woodworks + logging villages (Seonon) should bring more to the table. If you control the top producing zone of iron your faction would obviously have more access to iron crafts making low tier armies cheaper - if your faction owns logging + iron and smithies (translating into mass production of steel), than higher tier troops should be more common and less expensive to maintain; And basically the logic would fall onto that. As is it's just bonkers, htf a faction manages to finance and field a full cataphract army with zero weapon & armor production + access to horses!? Doesn't make sense. At most they'd have a small unit of it and it'd be very costly - do you get what I mean!? The entire game's built upon very illogical systems, it's much more arcade than it is any sort of RTS simulation or even RPG.
And that's just a simple and small example - to me a lot of thing should be added / changed. Furthermore, some extra details should matter too, like winter + snow should fk up your troops if they aren't geared for that - I'd likely try and see if it was possible to actually make the shoulder armor piece, at bare minimu, into a slot that dynamically changes depending on season + region and pull something off like needing warmth materials or gear in the inventory for it to work - so if you went to sturgia as Aserai during winter you'd need a small stock of furs/pelts or cloak/pelt shoulder armor in your inventory, otherwise overtime your troops would start getting wounded / dying from attrition. A same logic for Desert zones over stockpiling water and having heat resistant cloth (silk/cotton) in your inventory
Anyway, I sidetracked from my initial thought, what I was going for was to repeat something I said before with more detailed explanation, which's that if I was in charge or put in charge of the lead design of the game I'd bin most of it's current systems and remake everything from scratch due to those reasons, they'd accomplish similar things but would work completely different... I already gave some thought about it all and I have a pretty good idea of how to do it, yet I'm no Game Designer for them and even if I was I'd have to pitch it and convince "da b0ss" to actually do it, which's probably impossible at this point.