@maltfield Good question.
I’m not sure about the AJAX restrictions; I imagine that they are for simplicity’s sake and easily extended.
Regarding the API, it still doesn’t yet support attributes. A few weeks ago we at phpList Ltd. quoted a self-hosted user for adding this, but unfortunately the project took a different turn instead, and the attributes weren’t funded. Attribute support will be added, it’s just a case of when. If you are familar with Symfony and Doctrine you could look into this yourself in the repositories.
Out of interest the cost for our staff developers to comprehensively add this functionality to the API (without community PRs), along with various necessary technical plumbing, is approximately $4,500, and 6-8 weeks work. In the next few months we hope to make it easier for people who are so-inclined to co-fund such features, as a means of prioritising and speeding up frequently-requested improvements. Feel free to private message me for details.