X-Purpose: instant - so, another non-standard header to deal with.
And what about...
"When Google Chrome receives this status code, it will add your website to a blacklist maintained on the client. This blacklist will last the duration of that user’s browsing session."
Looks as if it may be better to detect X-Purpose: (anything) and return a 200 page with text something like, "You are using a stupid service. Drop google and Chrome and use Bing and Firefox instead!"
More damn work just to deal with $(*^ google!