I think anyone with moderate video editing skills could do my first option on their home PC to most people's satisfaction. Riker doesn't move position between the two shots, so with his head staying still, compositing his SD mouth movements on to his HD face would pose no issues. We've all seen far more advanced stuff on youtube that some very skilled "amateurs" have done at home.
It's a shame that doing it didn't occur to anyone at CBS. It occurred to me straight away. It would only have taken a fully trained CBS in house monkey an hour or two to knock something up.