There are summing bus modules out there. The Trident Series 80/80B/80C/70/Trimix consoles used bus modules that contained all the summing circuitry. I'm working on a Trimix right now, which is totally modular with a passive mainframe. The sub-master and master modules are identical in everything but pin-out.
I'm recapping it shortly and will be trying a few different chip upgrades on the summing busses. I've got two spare subs and one spare master which should make before/after comparisons easy. I hope to end up with a refurbished all original master section plus a couple of swappable fast-clean summing buses for when the Trident "crunch" isn't appropriate. Different concept than what you're talking about, but it does rely on the summing buses being modular units. HTH