I got to thinking about this and trying to take it out of mothballs and I'm starting to think it's a terrible idea for 4.x and should be postponed or abandoned unless and until we do something like what has been discussed elsewhere; having there be "one source of truth" (ZooKeeper has been discussed for instance). So I'll list out the issues I've thought about and if there are straightforward answers to them I'll be happy to reconsider.
Each issue is probably technically do-able, but the sum (and ones I haven't seen yet) totally scare me.
1> Traditional master/slave architectures. Let's say we change the schema (it'd have to be on the master?). How to get that to the slaves? Currently the confFiles directive has an explicit test and will not copy a directory. I'm not convinced it'd even work with relative paths and listing every file in the configset dir would be kludgy at best. And I think the confFiles directive doesn't work outside the "conf" directory for the core it's replicating anyway. I suppose the user could copy the configset directory to all the nodes in the farm, but....
2> The new REST API for modifying the schema. In non-SolrCloud mode, how does that work? Is it only allowed on the master (assuming we can solve <1>)? How to enforce?
3> Sharing the solrConfig object is also fraught with issues as discussed above. There's already the "share schema" option, so at least it's possible to have one shared schema.
4> How to get any changes reloaded in a master/slave environment for all the affected cores on all the machines? You'd need some kind of manual process of going to each one and issuing a new command "ReloadAllCores" or build in some kind of notification system. Or we'd need to require the user to keep a list of all the nodes and all the cores and script reloading them all. Nobody should be re-inventing ZooKeeper.
5> How to get any changes reloaded in even the non master/slave environment for all the affected cores? A new command? Periodic polling? Check every query/update request?
6> Sticky wickets I haven't thought of yet, I'm afraid, very afraid... Each of these is solvable, but considering the effort involved it doesn't seem like it's worth pursuing right now, at least my interest is disappearing.
And wrapped around this is that SolrCloud already handles most of the things I'm worried about, especially getting changes propagated to all the right places in the cluster. SolrCloud already has a way to reload all the nodes that take part in a collection. SolrCloud already has the notifications of changes to the config set built in (at least I think, if not it will).
My feeling at this point is that supporting this well would turn into a huge amount of work that would then be thrown away if we go to a "one source of truth" model in Solr5 (or even 6). And that actually using the capability would be fragile and complex. So unless I can be convinced otherwise, I'm going to assign this back to nobody and forget about it.