I had high hopes for this at 10.1 but have been unable to accomplish it. All I want is to do is serve a joined table out through ArcGIS Server and edit it preferably via ArcGIS Online.
Serving a feature class with a relationship class built on it has been the closest I have been able to get, but it appears ArcGIS Online does not support Relationship Classes. I could write some code to accomplish the task, but I'm looking for a more "Out of the Box Solution".
The "Pie in the Sky" goal would be for the Table to reside in a non-SDE SQL Geodatabase... but this doesn't seem possible without converting the geometry into SQL Spatial, since the Spatial Data Server (SDS) doesn't support SDE data and ArcGIS Server doesn't support non-SDE data.
You can put the table in a non-SDE database (SDS) and the feature class in SDS as well, if this is consistent with your framework. Then you can create a view in the SDS to create the join between the feature class and the table and share the service on ArcGIS Online.