Now I understand what you want to do.
In your first post you mentioned that if you right click the layer, your solution works, but you did not want your users to be forced to select the layer. Doesn't the Start Editing dialog do the same, if you have two layers from different worspaces, one being a shapefile and one being a featureclass from a read only SDE geodatabase, depending on which layer your user clicks in the first Start Editing dialog, depends on what they see in the second Start Editing dialog (if there is a second dialog).
If your user clicks on the shapefile workspace, you won't get a second Start Editing dialog. If your user clicks on the read only SDE featureclass workspace, you will get a second Start Editing dialog that says something to the effect of no editable layer, and that the layer or table is not registered as versioned.
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