Publisher hosted copies are in the pipeline, as I referenced in the parent comment. My choice of verbiage was a bit confusing it appears.
AMP's documentation seems to indicate that the LTS is stable only for one month (new features released via the same URL each month), and so is not compatible with SRI (see https://github.com/ampproject/amphtml/blob/master/contributi...)
You can specify a version (ie, https://cdn.ampproject.org/rtv/somenum/v0.js), but the AMP validator complains about that.