Keybase Pages Request: cannot specify a 404

In kbp sites, I want to be able to specify a 404 page, but it is not clear how to do it.

Is there a way? If not, I would like to put it down as a feature request.

Start by checking how 404 handling works for kbp sites and whether a custom 404 page is currently supported. Done means establishing the supported behavior and, if it is absent, defining the implementation scope and expected result for a configurable 404 page.

Written by the indexing model from the issue text.