The adsense ads are allowed on 404 pages provided it has some contents. But displaying the ads on empty pages is definitely against the google TOS. I do not normally display ads on such pages.
I think that this has to be settled by asking Google (how long does something like this take? has anybody done this?). I don't have time to write a long and friendly letter, but I should explain the situ a bit. In the interim, maybe we can reach a consensus. I'll post the relevant info when I get a response from Google.
I have written to AdSense support and asked them about the gray area - and the content ads seeming ability in my case at least to display relevant ads for the most-part. In any event, I switched to non content ads until I get the scoop for my type of error page. I'll write back here once I know for certain.