After talking to darbelo I've decided PLA is not going to attempt to track supported releases, at least not for normal development. It's too much of a pain in the ass, and nobody else is doing it. Sure it's best practice, but it's the kind of thing the whole ecosystem really needs to do together. If the HLL compiler is tracking Parrot HEAD because of a desperate need of new features and improved performance, then it doesn't make any sense for libraries not to be keeping up pace.
Darbelo made one particularly good point:
If we track HEAD the we'll run on 2.6 when it's released. We branch on the release day and do whatever 'release engineering' we need to do on the branch and tag a release when we're done.
That's it, in a nutshell: If we track trunk closely enough, we will always be ready with a release following Parrot's release. Spend a few days to prepare the necessary accouterments, then cut a tag and continue on with normal development. Our releases can be tied to Parrot's supported releases, but our ongoing development will target trunk as closely as possible. I don't anticipate that we will want to cut a PLA release every time Parrot does, but if we follow a 4-month schedule and follow supported releases that should be just fine. At least, fine for now.
PLA is going to track Parrot trunk and hopefully be prepared to cut a release following Parrot 2.6.0. That gives us about 4 weeks to get all our ducks in a row (and our Kakapos too!).