That's super interesting! Thanks for sharing! Do you mind sharing a bit more detail around the types of information you're hoping to extend the Connect content usage with? Is this granular-level developer-implemented usage metrics? Something like Google or Piwik analytics?
I can't think of a unique identifier off-hand, although if there was I would suspect it would be in Environment Variables, the server PID, or some other session data (i.e. every Shiny session gets a unique ID).
This Shiny app may give you a better picture of the types of session info that are available. https://github.com/colearendt/shiny-session-info
Please keep us posted on your progress, though, as this seems like an interesting use case that it may make sense to see if we can provide better support for in the product.