This is part of a larger direction we have been working towards, which is essentially making ZooKeeper the truth.
With the current SolrCloud, you cannot do this like you have. The Core Admin API, and pre configured cores, and the collections API all need to work in concert. That makes this approach a complete no go right now.
The path I have been working towards with the Collections API is a new mode where everything is handled by the Collections API. In this case, it will not be valid for users to try and mess with things at a per core level.
In this way, the cluster can truly match the truth in ZooKeeper because both the nodes and the Overseer can work together to keep the truth enforced. This includes things like being able to easily change the replication factor for a collection, or add a new host that automatically gets used depending on your settings. You should not have to address individual nodes to manage collections, nor should your replicationFactor stop mattering simply because you added another core via core admin. To me, this is the ultimate cloud situation. The system needs full control. We can add the ability to override or prefer certain things, but in general, we want to get to the point of optionally have the cluster mostly managed for you given some simple directectives. Of course, I think it should be implemented as a bunch of option features. You should also be easy to really lock things done unless you manage things manually.
All of this requires we have a mode a user can decide to use (the collections api, perhaps with an option for back compat) so that we are in control of everything. We know when a collection is created and deleted - it won't be able to just pop back into existence.
Until we have this special mode, the way that we had to build this, lots of historical reasons, we currently have to support what we have with pre configured cores and core admin and the collections api. This is silly form a user perspective though. It can all be done much nicer with just a collections API that doesn't have to be directed to any single node.
Doing what you want to do in a back compat way is not some simple fix. We have been working towards this for a long time now - if you could just slap in a band aid and make it work like this, I would have done it a long time go.