Another way of dealing with pole piece/string alignment might be to look into the Gretsch "Space Control" bass bridge. I think the post centers are 74mm apart but don't know about any other dimensions, such as main/max spacing adjustment limits. Also don't know whether they sound any good, as I've never had one (maybe somebody else here has had first-hand experience?). Just thought of them because they are one of the few bridges I've seen that are intended to work in tandem with a trapeze-ish style tailpiece and allow for adjusting string spacing. Of course, because they don't have individually-adjustable saddles, you would loose precision of intonation adjustment to gain string-spacing adjustability, especially if you didn't also use the "floating" base, so they actually might not be a practical alternative:
I had a Tru-Arc bridge on a Gretsch guitar. It also lacked individually-adjustable saddles and sat on a floating bass which allowed me to set it up without any particularly egregious intonation issues.
Unfortunately, most of the other bass bridges I've seen that allow for adjustment of both intonation and string spacing are replacements for either Fender "bent-plate/barrel" or Gibson 3-point type bridges, not the Tune-O-Matic.
Only other thing that comes to mind are the bridges on those old 1970s, Japanese import basses, some of which had fairly narrow string spacing. Might be tough to find something like that that was detached from the original bass and just floating around, though.
