I'm hoping that public nodes will run most or even all the current HAF apps.
As for public access to a HAF server for development purposes, as you suggest, this may also be feasible, but it is probably best to run it on a node that's not acting as a regular public Hive API node. We're basically planning to do that with one of our HAF servers (well, we've done it already once, planning to put up a new one soon) that uses the query_supervisor as a rate-limiter to protect against rogue queries. A multi-regional one might also be useful to reduce latency during app testing.
I am planning to run HAF apps that will definitely be beneficial to the community on all my nodes (over 8 nodes worldwide) so I am hoping it would both help the availability of the apps and the capacity it can handle. (Balance tracker so far has been the most requested one by a lot of people, so that is a must at the very least :))
Looking forward to the publicly available release (in terms of ease-of-use and access) for the aforementioned HAF apps and query supervisor (I believe its still WIP) so I can do a major upgrade and update sweep in one go.
The public release should be very soon, we've already been begun testing release candidates and we're also finishing up development of docker compose scripts for easy deployment of all the needed dockers :hived/haf docker and dockers for all the various haf apps, plus some optional monitoring tools (pghero, etc).