I imagine that tons of their user base (especially for small hobby sites that only occasionally, if ever, see high traffic or run background tasks) look for this, and yet there's nothing pointing to it other than old posts on the mailing list. At least put information and resources for autoscaling in the Docs section of the site.
Acquiring resources only as long as you need them, and then releasing them when you're done, should be the standard service - complete user control should be the advanced feature set. They already do this for websites on a single dyno: they spin them down when they're idle for a while. But they don't do it with workers (what effect does the latency of starting a backup job up really have? it's already running in the background).