I disagree with this being a "Won't Fix". I am of the opinion that the _show handler has correct behavior here (and by extension, that the _update handler's swallowing of slashes in docids is incorrect behavior). The patch supplied above just duplicates the docid extraction routine from handle_doc_show_req/3 to handle_doc_update_req/3.
My couchapps use slashes in docids to namespace documents, allowing me to use rewrite rules to force _show, _update, and _list handlers to only handle documents in a specified namespace. If slashes are not allowed in docids, this would not be possible (and would be a massive showstopper for me).
That being said, I do encounter some woes when it comes to trying to get attachments out of namespaced documents with rewrite rules (there's no way around escaping the %2F there), but that has nothing to do with the behavior of _show and _update handlers.