I cannot comment on the WMS discussion.
The question I do have thoughts on is whether or not you can use the REST API in order create a ArcGIS Server map service. I think that the answer to this question is no. IMHO, the map service is focused on providing access to the information which is published in a map document. And, in fact, that level of access you have to that information (i.e. fined grained ArcObject interfaces) is being greatly reduced in 10.1. I only mention the last point because it is a way of saying that server is getting more focused.
So... the alternative is to create your own RESTful service in which you can do what you want, e.g. communicate with the SOM. BTW, the service you create is external to ArcGIS Server.
I struggle with this exact problem because we need a service which responds to requests from an external system in order to do operations like import/export data, create a new SDE version, reconcile/post, etc. These type of requests do not seem to fit with ArcGIS Server. My thought is that such requests require the development of a custom service which is based on ArcEngine. You cannot use ArcMap because ESRI does not support it running in this manner.
I would very interested in hearing comments/thoughts from other users and ESRI themselves.