In that photo of Lucas surrounded by models and props, you can see several KENNER DIE-CAST TIES on a table. You know, the toy that duplicated the larger Kenner toy in a much smaller size, right down to the battery compartment. :confused (I think the solar panels were more accurately proportioned, though.)
It's possible they were only used for animatics, although I can imagine them being used for distance shots, TIEs teeny-tiny waaay back there in the background.
I suspect the MPC X-Wings were used for pyro--perhaps the "she's gonna blow." guy. I've never gotten around to analyzing it, though.