mw.org seems to be a reserved address. And AFAIK, the WMF don't own it
mw.org is already registered* Name is reserved by Registry_Policy >>> Last update of WHOIS database: 2017-02-19T00:45:00Z <<< Access to Public Interest Registry WHOIS information is provided to assist persons in determining the contents of a domain name registration record in the Public Interest Registry registry database. The data in this record is provided by Public Interest Registry for informational purposes only, and Public Interest Registry does not guarantee its accuracy. This service is intended only for query-based access. You agree that you will use this data only for lawful purposes and that, under no circumstances will you use this data to: (a) allow, enable, or otherwise support the transmission by e-mail, telephone, or facsimile of mass unsolicited, commercial advertising or solicitations to entities other than the data recipient's own existing customers; or (b) enable high volume, automated, electronic processes that send queries or data to the systems of Registry Operator, a Registrar, or Afilias except as reasonably necessary to register domain names or modify existing registrations. All rights reserved. Public Interest Registry reserves the right to modify these terms at any time. By submitting this query, you agree to abide by this policy.
We could speak to ICANN etc.
Though, we actively removed redirects after we forced HTTPS, as we'd need separate SSL certs (which cost money)... Or LetsEncrypt for production redirects (which we've not done yet)
For this reason we've not use any of the wiki TLD we were actually given
It is fairly often; however it's mostly on IRC etc where we don't need to write the full thing, and we're probably just wanting a simple distinction between mw, wp, wmf et al (for people who know what's what anyway)
Self signed definitely not, as that would require anyone that wanted to use it, to have it installed locally, or bypass the exception. Which doesn't really help.
The question is whether Ops et al are "happy" at using LE for this sort of thing
One- and two-character .org domain names aren't available for general registration. See, for example, this press release from Public Interest Registry (the registry for the .org TLD) when they made 94 of them available for the first time: https://pir.org/org-releases-94-never-before-registered-one-and-two-character-domain-names/
We could petition Public Interest Registry to let us register mw.org, but they would be very resistant and we would have to make an extremely compelling case. I don't think "MW is often used as an abbreviation of MediaWiki" is going to be enough to convince them.
The good news is that no one else will likely be able to register mw.org, either, so it won't be used to cause confusion.