Here may be a guide for getting this in front of the right people.

https://meta.wikimedia.org/wiki/User:Halfak_%28WMF%29/Wikipedian_recruitment

So, in technical terms at the varnish level, by the time we're working on this we'll have mobile and text traffic flowing through a single cache cluster anyways, with the primary distinction being that the incoming domainname (m-dot or not) determines setting X-Subdomain: M or X-Subdomain: Zero or not at all for desktop, then the m-dot hostname is rewritten to the desktop hostname, and the cache is fragmenting itself on the X-Subdomain header's presence and a Vary on X-Subdomain.

Currently, we're also clearing X-Subdomain on inbound traffic initially, so that only our own code makes that determination.

So, what we could do is have JS code do the 0.1% filter in the browser, and if so send its own X-Subdomain: M, and not filter that on inbound so that it actually works (and they'd get mobile content for a text hostname, and all the cache stuff would work fine). Or alternatively, we could do something on the varnish level if it was per-client-IP (as in, hash the client IP with sha1 and get 0.1% by regex matching the last 4 digits of the hash on ff[0-3][0-f], which would be 64 IP hashes out of ~64K, or something similar in nature).