I could see RD Jr doing a cameo appearance as Stark. I don't think he'd be above a TV appearance to help the larger franchise. No IM, just one or two Stark scenes.
Totally agree. I don't think either actor would see it "below" him to appear on TV.
If they really
wanted to make it easy/cheap, I could imagine them recycling movie footage of the actors (on a video screen even) and just having SLJ or RDJ do simple voice overs would do the trick.