That is an interesting question.
BOK only has about 50-60 systems and you can see how clustered they are. I think...I think I'd handle it by scaling out and creating supplementary maps. Kind of like zooming in. The Core map would present a broad Overwatch, a sort of snapshot of the forum populace, if you will. It would hold a fair bit of room for expansion, for when new players join the forums and create their own planets/systems. Then we take a look at a Star Empire, say, USA (being the biggest in your list), and simplistically divide it down into smaller, more manageable chunks. We'd want each map to contain 100 systems or less, I think. It seems like it would be ok for powerful systems to look more clustered. Theoretically, it would give the viewer the immediate impression that this group is kind of a big deal. I'm no astrophysicist (obviously) so whether this makes astrological sense is beyond me. But it works for the narrative. So anyway, we break USA down into 5 sectors, as opposed to systems, represented on the Core Map. If you look at BOK's political boundaries, this would be roughly equivalent to the Hiveworlds or the Independent Fringesystems (and it really is all about how you frame them...notice the weaker shape around the resource starved rebels). Then the USA gets 5 maps representing the actual systems (planets are assumed and can be discussed in the narrative of the particular faction...since most of them will never, ever be mentioned). ...Hm...This part may not even actually be required. I can just see the maps stacking up...3 for this guy, 5 for that guy, 12 for that asshole over there. But, like the planets, most of them are filler for the narrative. They're like requisition or energy points in a supergalactic RTS. So most of them wouldn't need to be mentioned. I could see sectors in longstanding contention being laid out, key points along an Empire's galactic frontier. I could also see a map of a player's home system (but that strikes me more as a prestige item than anything else...it's cool, but practically almost as superfluous as the other systems).
So, basically, what we're talking about here is taking that big number of systems for each empire, dividing by 100 (or so), and displaying their relation (in Sectors) to one another on the map.
The Trattorians could have two sectors (one big, one about half the size of the first), the Scythians would get 4, USA gets 5, Assyrians would get 3 (two big, one half). The other players would be grouped together in various pocket sectors, with only the big empires claiming such vast swaths of domain. So here's where it gets interesting to me, or where it all completely falls apart. New players to the Galaktik Metaplot would no longer have the option, logically, of naming their system and randomly choosing to be a big, badass empire (unless the community agreed that their army/story was just too cool not to include and called for galaktik expansion of the map). They'd have to fight for it. That unfairness, and the sheer bloody violence of it, strikes me as quintessentially BrikWars. On the other hand, it might turn newbies off. What's more, if the story (told presumably mainly through Forum Battles and Reports from the Field) stops moving forward, it could breed resentment. I've seen that before, on other forums and in RPG organizations. But then back on the first hand, the Forum Battles are the most popular thing here, so I don't know. I hang about but I don't rightfully have enough knowledge to give a real informed opinion on that situation.