Not the core API, but a simple custom timezone can be used to implement TAI. I worked on a POC for it, but you can totally test this with the polyfill right now.
I attempted to test your library's behavior in this regard, but I do not think you are correctly implementing the proposal's TimeZone. TimeZone.getDateTimeFor() is supposed to take an Absolute and return, in your case, the time that absolute represents in TAI (that time, in the TimeZone). But this:
var one_before = new Temporal.Absolute(915148799n * 1000000000n);
console.log(one_before.toString());
console.log((new Temporal.TimeZone('UTC')).getDateTimeFor(one_before).toString());
console.log((new TAI()).getDateTimeFor(one_before).toString());
1998-12-31T23:59:59Z
1998-12-31T23:59:59
1970-01-01T00:15:15.148768
The other direction (getPossibleAbsolutes) is similarly effected.