I don’t know why it didn’t occur to me sooner, but I just put up a quick shortcut for actually retrieving any public write.as post’s body text via the API: Get Write.as Post Text
By default, it just copies the text to the system clipboard, but - as I noted on the RoutineHub page - you could append an action from any application(s) that saves text. Here’s my personalized example for Drafts:
This one does not require authorization, FYI.