At the moment there are is the Actions add-on which makes it possible to display transitions on a portal. The add-on has to be configured with a "Technical User" who is a JIRA user and will execute the transition, regardless of who is logged in to the portal.

In my workflow I have transitions with various custom conditions, validators and post-functions (provided by add-ons, script runner scripts, developed ourselves, ...). However, it does not seem possible to figure out who is actually logged in in the portal and do something with that info.

In the code snippet below, both the loggedInUser and caller will refer to the "Technical User".

ApplicationUser loggedInUser = ComponentAccessor.getJiraAuthenticationContext().getUser()

String caller = ((WorkflowContext) transientVars.get("context")).getCaller();

For postfunctions, for example, a simple solution could be to add "portalUser" to "transientVars". Then it can be accessed via:

String portalUser = transientVars.get("portalUser")

I noticed that work on JSD-40 has started, but I'm not sure if that will also cover this suggestion.