There are some tricks using the XML file from FCPX and modifying the framerate, but I have not tried it and it seems a bit convoluted. You may be better off using Compressor to export your master file as it has default settings that include 12 FPS. Also - as far as I know, most old movies were typically at 18 to 22 FPS, but that 's probably irrelevant. As far as plugins go, check out Luca VisualFX "Old Time Movie" available through FX Factory. It has an amazing set of parameters to provide that old time look, including the old title cards.
Jonathan Levin wrote: Just a wild guess, but could the Retiming editor be used for this?
Thought you might be able to skip every other frame in a 24p timeline with 200% speed, alt+g and then slow the compound down to 50% - no frame blending or optical flow. You would have had a 24p output, but with the same look. Unfortunately, the motion then looks like the original clip. So nesting doesn't do the trick. Then I thought of re-importing the sped-up compound and again slowing it down. That's what I had done with the first few minutes of LOTR ("the world has changed ...") to see how 12 fps compare to The Hobbit's 48 fps. Try it just for fun: if 24 fps looks better than 48 fps, then 12 fps look epic. A good recipe for wedding videographers. Don't use higher frame rates for your bridal kiss slomo, let 12 fps rock!