Going back to the comment about the Media Player and Calendar. Both could certainly be replaced, but the question is really whether it's worth investing the time and effort to do this as they will presumably be completely replaced in the next Harmattan/Meego release.
Having source available would make this significantly easier and cheaper (in terms of time) to do. OTOH I understand the point about the UI being Nokia's differentiator, though now that Maemo is an old platform, I'd have thought it's less important.