To my mind, there have to be Updates rather than updating the original post. [...]

I'm also leaning towards that, with one, or possibly two additions:

1. I would update the original proposal post to link the additional updates, to be easier to find (or manage this through the interface like Kickstarter)
2. (optionally, for "extra points") I would link each newly proposed feature / improvement of an existing one from the original proposal post to the update post(s) where its progress is discussed.

I think this would be a good post for the SteemDevs community

I haven't thought of that. Initially I wanted to add this in SteemHelp community, but eventually I just added it on my blog.