hi, 
grab video from camera in real-time
no problem in PDP ( pdp_v4l from PDP ).
periodically save it to a file(s)
no problem in PDP ( pdp_rec~ from PiDiP )

chroma key it as it is displayed
    

  
kind of ( you can use pdp_compose from PiDiP )
or use objects from Gem that can then be displayed in PDP
( using pdp2gem )

  
concurrently display previously-saved video(s) behind the current real-time video
no problem in PDP ( pdp_qt )

  
apply diverse effects in real-time to the previously-saved video files that have been loaded in
of course, a lot from PDP and PiDiP
( http://ydegoyon.free.fr/pidip.html )

    
vary the amount of these effects according to gradiant input from a motion
sensor
sure if it has a midi or net interface.
i know people commanding PDP/PiDiP with midi faders
or USB gamepads.
also, PiDiP does some motion detection ( pdp_ctrack, pdp_mgrid, pdp_shape )

just a detail, it's only for linux & partly for OSX ( to be improved ).

cheers,
sevy