It surely would not be conformant to Plan 9 systems, but to the protocol?
As far as I can read intro(5), it explicitly excludes slash as a valid character for the Plan 9 OS, but it also explicitly states that "the protocol has no such restriction".
Be patient: I'm asking because this could be a typo in the intro(5) man page, or in my understanding of its phrasing.
Still, using this protocol "feature" to enable atomic directory change could be useful in my use cases, but I don't want to build yet another 9p2000 extension.
Giacomo