We have many examples of nodes that function in a block now days (foreach block, compile block, etc.). Is there a way to make HDAs that work this way, with a start node, an end node, and a nice outline around any nodes the user places between?

This is a popular RFE (feature request) but no it's not available outside of the available tools that draw a convex hull around their contents.

is this possible yet?

I posted about it on a few beta programs, but it's not available yet.

Out of curiosity, what kind of data would these node blocks have to fetch and output ? And what kind of application would this help for ?