Kirk,
Yes, I've made quite a bit of progress in editing via RIA Services. In my old post that you linked to, I was asking about restricting the domain data source to a spatial query. The idea was that I draw a shape, and my DDS populates my DataGrid with those features that intersect the drawing.
I've pretty much kept that same concept, but I no longer work with the DDS in Xaml to perform this function. Here's my workflow:
1) I have a draw function that the user selects, while it's active the user draws on a GraphicsLayer. When the 'OnDrawComplete' event fires, I take that graphic's geometry and use it in a QueryTask to get the parcels that intersect the drawing.
2) On 'QueryExecuteCompleted' I loop through the returned features and grab a field which contains a GUID. That guid is used to load that particular parcel into my DomainServiceContext (created in code-behind) using a custom Query function in the DomainServiceClass.
3) When I"m done looping through, I set my DataGrid's Itemsource to be the DomainServiceContext. The Grid populates with all the information which is then edited by the user.
Other than that, when I'm looping through the features, I take that time to add new graphics to a GraphicsLayer so the user can see the selected parcels. There's also a DataGrid_SelectionChanged event that highlights the selected parcel.
So yeah, I've written about ten editors using this method, or very similar to it so far, but all of them are mainly editing tables which are joined to feature classes. Anytime I've had to edit geometries, I either use FeatureLayers, or if the edits are rare enough, do it in ArcMap and let the user edit the attributes through a table editor.