Even better solution: embrace the RoutePart's editor.
Placement.info: Change the line suppressing the Parts_Routable_Edit to Content:before.5 and remove/suppress the Pars_Routable_Thread_Edit line.
ThreadPartDriver GET Editor method: you can remove the Parts_Routable_Thread_Edit ContentShape from the CombinedResult.
Views/EditorTemplates: Delete the Parts.Routable.ThreadPart.cshtml view and create a Parts.Routable.RoutePart.cshtml view.
and here's the kicker that got me hung up for a while:
Module.txt: Add a dependency on Routable
I've created a fork with all these changes. I want to play around with it a little bit more, but man was this a learning experience!