I think I read somewhere that the council membership-related minor titles are modable, which to me implies that there's a tag applied to them that lets you mark a title as one that provides a seat on the council on top of the obvious ability to change the number of said spots on the council if modders want. This, combined with the fact that the team making this mod has already proven that minor titles can automatically be given out to the holders of the kingdom-tier titles under the King on the Iron Throne after a megawar ends (the Warden titles), suggests that it'd perhaps be possible to have each kingdom-tier title have a minor title associated with it that gives a seat on the council and is automatically given to the proper holders after a megawar, or make it so that the current holder of a council position prior to a war is given a specific flag so when the megawar ends, a specific event automatically gives them their previous council position back. On the other side of things, perhaps the Warden titles could be used for the same purpose since each is traditionally held by a LP anyways? This would provide a guaranteed seat on the council for four of the LPs, and with three "Small Council Member" minor/honorary titles, that's seven of the eight LPs accounted for if they all have to be on the council. The only problem is you'd have a very large council as the ruler of the Iron Throne no matter which option is used.
Edit: I just checked the files, adding the line "is_voter = yes" to a minor title makes it give a vote on the council, so keeping them happy would, in theory, be as easy as adding that line to the small council honorary titles, which would lead to 8 council members on its own.