I just pushed a small node.js script to add an entry to the update.json manifest given a signed XPI. It’s here.
Right now, the biggest problem to automate a developer-hosted beta channel is that the signing of the add-on package (required to distribute the extension) must be done manually on amo. The current add-on signing API fails if the extension is available publicly on amo, which is generally the case.
I had a look at this signing API but it seems the/some changes are to be made on the server side. I hope someone at Mozilla can take care of this before beta channels are removed from amo.