Just another reminder: I work mostly with content and human’s way to use content, and I’m a webmaster, sysadmin and admin of everything just because I’m poor and small fish…
But — can the issue here is so simple that because Discourse is kind of a-typical 
web-solution it is harder or close to impossible send JS-based content when serving html status error 404 that needs static content in the meaning how a client sees it? I don’t know at all what W3C, RFC etc. says.
No? Not even close?