Find out a tab or portal's relationship

[amcgilly]

I would like to be able to programatically find out the relationship a tab or portal is based on.
In other words, I'd like something like this:

myPortal.relationship //returns the name of the relationship the portal is based on.
myTab.relationship //returns the name of the relationship the tab is based on

Am I right that that's not currently possible (except by some indirect approach involving functions like getTabNameAt() etc.)?