Looking at the portfolio (and this may be the case for some other areas too), I see a situation where a given component calls the portfolio subsystem, which links to the plugin types (portfolio). The plugin types (Mahara for example) link externally.

The issue I see here is that only the component (let's use forum as an example), knows what data it passes to the subsystem, and which subsequently gets exported externally. The subsystem and the plugins just pass it along, and don't know this. To me, it sounds like we need a way to say something like "Link subsystem with this data" - which we currently don't have with the link subsystem class. Right now, we don't / can't describe metadata about the fields sent externally through this flow: forum -> portfolio subsystem -> mahara plugin.

I'd think one way to address this would be to allow passing in fields to link_subsystem.