[imuli]

I did this for my systems a while ago. You can grab <https://imu.li/TAI.zone>, compile it with the tzdata tools, and stick it in /etc/zoneinfo. It is unfortunately unable to keep time during a leap second.

In theory, if you keep your clock set to TAI instead of UTC, you can use the /etc/zoneinfo/right timezones for civic time and make a (simpler) TAI zone file. I learned of that after I'd created the above though, and I can imagine all sorts of problems with getting the NTP daemon to do the right thing, and my use case was more TZ=TAI date, as you mentioned.

There's a contentious discussion on the time zone mailing list about adding a TAI entry. It really didn't help that DJB was the one wanting to add it and approached the issue with his customary attitude. There's a lot of interesting stuff in there though - like allegedly there's a legal requirement in Germany for their time zone to be fixed to the rotation of the earth (and so they might abandon UTC if it gives up leap seconds).