I like the idea, another plus of it is that it would help balance out the advantage of starting in the center by blocking one direction of expansion for those races. Assuming it's big enough anyway.
The supermassive black hole available in the editor is 19 tiles in diameter (271 tiles are covered). Adding just one of these to a map removes roughly 10% of a Tiny map, 5.5% of a Small map, or 2.5% of a Medium map from play, constraining empires to be within a 21- (Tiny), 31- (Small), 51- (Medium), or 66-tile (Large) wide band around the edge of the map. The width of the band on a Large map is only 5 tiles greater than the diameter of a Tiny map and 10 tiles less than the diameter of a Tiny map on a Medium map, while the width of the band on the Small map is just one tile greater than the radius of a tiny map. That suggests to me that the supermassive black hole we have is more than large enough on those map sizes to do what you're asking. I find the assertion that this is a positive thing questionable, however; the differing quality of starting positions is part of the game, and the smaller map sizes are the ones that least need a reduction in potential expansion directions to counterbalance the advantage of starting in the middle anyways. The larger map sizes are increasingly unaffected by the addition of a supermassive black hole in the center, and moreover to achieve the effect you seem to desire would necessitate throwing away a significant fraction of the map especially on the larger map sizes, unless you want to remove dead center from the somewhat advantaged region and instead have an advantaged ring.
is it possible to script that sort of thing through the editor?
As far as I am aware, the editor only allows the creation of mostly-fixed maps (the campaign maps, for example).
does it expose the algorithm to create the spiral for tweaking at all?
Not to my knowledge, unless you count "manually adding and removing stars, planets, etc from the map" as "tweaking" the algorithm.
