Possable, but not too likely. Valve seats are pressed in pretty good. Did you Ford Mechanic friend put your car on an ignition scope to check it out or is he just guessing at this point. What exactly is the code and the description for it?? Does it specify a lean misfire or ritch misfire? also have you tried doing a compession test or cylinder leak-down test?? One of those will tell you right away if there is a loose valve seat. Ask him what made him come to that conclution. Because if he just guessed loose valve seat I want to make sure I never bring my car to him, because that would be a hella exsensive repair, not to mention if he got the head off and noticed the seat was ok.
I´m sorry if this sounds a little odd, but does the car actually feel like it is missing? I have a 94 GT and an 02 ZX2 and if you a dead cyclinder in either it is very noticable. If it is something that isnt happening constantly I would tend to dismiss the loose valve seat idea altogether. That and a loos valve seat is very unlikely to happen. I would also check the fuel delivery - injectors and wires. you can narrow down where to look by running a power balance to locate the weak cylindar.