I currently run SD on an iPad and on a Nexus 5 phone which I stick on the side window for a "HUD". It isn't easy doing route changes etc. on the phone, so would it be feasible to link the two together in flight so I could make changes on the ipad and it would be reflected immediately on the phone?

We add a whole bunch of file types when the user picks Share, from the native .flightplan file to HTML briefing output to a GPX file and others. I'm not aware of any more we can do to encourage other apps to display themselves when confronted with such a list, but am open to suggestions.