Would it be possible to have older versions as a beta option on steam?

Technically we could do this, but it would quickly go out of hand. We would need to make a new branch for each new version, and aside from that being impractical I don't think Steam will let us make endless branches. If you've bought the game through Humble you could keep your old downloads, so that would be the best way to do this right now.
That said, there is some merit in having old versions accessible. We have all the previous releases stored here internally, we'd just need a better way to make them available. So if there's enough need, we could see what we can do about this. Which leads me to ask:
Is there a particular previous release you're looking for, and why? If you need it fast, just send me an email and I can send it to you immediately.