I looked a bit into this so I guess I should claim it.

As far as I can tell, there’s no “silver bullet” that will let us implement this easily. The DB load group is specified when calling getConnection() or getConnectionRef() on an ILoadBalancer, and currently a lot of places in our code directly call those methods, and (as far as I can tell) always with the default group. Some classes extend the DBAccessBase class, which has special support for defaulting to a certain database domain, but not to a certain database group:

protected function getConnection( $id, array $groups = [] ) {
	return $this->getLoadBalancer()->getConnectionRef( $id, $groups, $this->dbDomain );
}

The LoadBalancer and LBFactory classes have a defaultGroup parameter that can be set at construction time, but there’s no provision for changing this parameter later. I also couldn’t find any way to get an “alternative version” of a load balancer (factory), so the idea of “let’s create a copy of the main load balancer, but with a different defaultGroup parameter, and inject that into various services” also falls flat.

So I suspect the solution to implementing custom DB load groups will be tedious but straightforward: add an optional $databaseGroups = [] parameter to various services that take an ILoadBalancer, similar to how some of them already take a $databaseDomain = false parameter (e. g. DatabaseMatchingTermsLookup). I haven’t found any better way.