I think it is possible to "fake" a Jacobite army from medieval figures, but you have to be selective, and it will require some "light" conversions (head swaps, converting mail to fabric, turning pants and leg armor into bare legs, changing shields). I made a few once from the Airfix Sherriff of Nottingham set. Differences in swords can probably be faked with paint. However you will probably only find a few figures per box that are good candidates for these conversions.
I converted Esci Muslim warriors, British grenadiers, Robin Hood and I think some Ancient Britons into a Jacobite army 20 years ago, but I'm talentless at this sort of thing. I'm sure someone more able could do a good job with the Esci figures.
Speaking of Jacobites, it's September: anyone got the GM ones yet??