Is there any method to adapt S16mm lenses to the EX3? And if there is, what would the magnification factor be? So far there seems to be slim pickings in the 1/2" native format.

Seems likely, as there is a fair bit of back focus distance to work with on the EX3.
Magnification is always a factor of the focal length, so a 50mm lens made for a 35mm stills camera will give x5 or so magnification on EX3, and a 50mm lens made for a 16mm camera will also give x5.
So you are always going to have a problem at the wide end, as in S16 lenses you've basically got a Canon 8-64/7-63or Zeiss/Optar 8mm and that's about it, not that wide on EX3.
Drop Les a line at les@lesbosher.co.uk, he's a font of knowledge on these matters!
Steve