Status: speculative proposal partly worked out, dissatisfied with results
The rules in the Player's Companion for spell-like proficiencies cap out at 6th level, with a 1 turn casting time once per month. That's more frequent than I really want, but let's see what we can do with that.
The scaling on "summons creatures totaling X times caster's HD" seems linear, at 75 spell points times X. We get about 60 spell points to work with. Duration of "Until one specific task is completed" sounds like a perfect fit, with x2 cost, where that task is "the conquest and looting of Place Y". "Comes by own locomotion from nearest lair" still works and gives us x0.66. I'm OK with restricting this further than the general "humans and demihumans" class of monsters from last time; this just calls a subset of Men (Bandits and Nomads), so we might be able to justify the x0.7 "specific monster" modifier for that. The 1HD max remains in effect for x0.15, and no modifier for special abilities. If we wanted to do cheese, we could take the "spell takes 1 turn to cast" and "can only be cast once per week" modifiers, but that seems against the spirit of things. It would get us almost 50% more barbarians though.
Ignoring the potential cheese modifiers, we get 75 * X = 60 / (0.15 * 0.66 * 2 * 0.7), which yields X = 5.77. So we can get almost 6 barbarians per level, which is... somewhat unimpressive. If you're a 10th level barbarian chieftain, 58 dudes is not enough to pose a credible threat to civilization. Even if you're a 14th level barbarian, you only get 81 dudes. The scale of the battles that you're fighting has probably quadrupled, but your horde only scales linearly.
(On the other hand, we might also consider a brigand camp leader who has about 200 total followers on average. In his case, an extra 60 men is pretty darn good! So we're running into the divergence between what ACKS expects of high-level characters vs what B/X expects of high-level characters)
Heck, 6 dudes per HD is the same effective multiplier that we got out of Call Allies as a spell like ability (which we could cast more often because it was lower level). The big difference here (and the one that we're paying out the nose for) is the semi-indefinite duration. Looking at Prepared Invasion in Crusader Kings again, it lets you build up troops for two years. If you cast Call Horde every month, building up troops who will hang around until it's go-time, for 24 months, we're talking about around 1400 men (48 platoons or 12 companies). Those are "taking a borderlands county" numbers. Of course, in the two years that you've been announcing your intent to invade to your fellow barbarians, the count has probably gotten wind of it, made some allies, and hired up to his ability to support troops. Still, it's a good start. These are also very much not "conquering the Kingdom of Scotland" numbers, like one of my past Prepared Invasions in CK2, so that's still a bit of a letdown. Maybe it needs to scale up exponentially, like troop numbers do in ACKS, doubling the men per month for every level after 9th, so that a 13th level barbarian massing troops on the border is something that worries domain rulers of a comparable level?
Having to build up troops for two years after you hit domain level is kind of lousy. My campaigns almost never make it to domain level, and they certainly don't make it another two years of game-time after that. Even if they did, is the target of the prepared invasion that you declared two years ago still going to be relevant, or will you have outleveled it?
There are plenty of other wrinkles here too. Do you have to pay the horde while they sit around? Do they just expect a bigger share of loot? How does supply work? Will they fight in your defense if you're attacked while they're mustered but before you launch the invasion? Why don't 11th level wizards research this spell and cast it every day, outclassing barbarian chieftains in their ability to muster troops by a factor of 30? Can you run out of recruitable barbarians in your nearby wilderness? Do multiple chieftains each working on a prepared invasion compete for available barbarians? How does this stack up against just using the domain-wide mercenary recruitment rules in Campaigns? If you call nomads, do they bring their horses for free, or do the horses count against your summoning HD limit (this is also a problem for Call Allies)?
Maybe Prepared Invasion is the wrong model? Maybe I should be thinking of building up hordes more like the CK2 Subjugation CB?
So anyway, I'm not totally satisfied with this. I do think something like this is important to making "domains are treasure that you take from other people" work for characters who are uncivilized / have poor access to markets, but I also think that at some point I will probably break with ACKS' assumptions around what peak population density / civilization looks like (in favor of less / smaller civilization, with probably duchies as the largest political unit), which would help here.