Part of the problem with Hyborem is that the AI sucks with Mane usage.
For the AI ... either get a random +1 population point per "mane" and don't even bother with manes, or give the AI free upgrades for manes- but that may be OP.
I think the way that the code works currently is that EVERY mane for the AI becomes a useless unit that you must pay to become useful.
I have seen an AI with roughly 100 regular units and 300-1000 manes. On what I think was diety, mind you, but still sucky.
Currently the AI gets a (permanent) mane for every mane, meaning its always going to be a unit. Honestly though, I'm not sure about in Naval AI. I thought my test game was in naval AI, but its been so long ago ...
I see 2 solutions.
#1. teach the AI how to use the "add to city" spell for manes. And tell it to do this 99.9% of the time. (unless it has less than 2 units per city, which is unlikely)
#2. Have a random infernal city gain +1 population (if AI) and circumvent the mane production completely (again, only if its under the control of an AI)
---> I could see both of these, though mainly #2, producing some potential Bugs for multiplayer.
I'm not really sure how close the FFH modding community is to solving his problem (meaning the collective code libraries of all mod-mods), but I suspect that it has not yet been completely solved.
Also in the current code (at least in .41n) i seems that a point of population IS added for some fraction of the # of manes created. If that code is indeed there, then perhaps that fraction should be upgraded into a 1:1 ratio. Then of course, the AI should be taught to delete their mane units once completed.
So possible solutions (assuming that manes are going to be produced regardless, lest we produce even MORE bugs)
#1. increase passive Pop growth to 1:1 with mane production and teach AI to delete all manes.
#2. Teach the AI how to settle manes into cities 99.9% of the time, while reducing the "passive" pop growth to zero (or some decidedly small fraction as a 'buffer boost' to the AI)