The main reason that I believe that a lightly loaded 738 (a premium-heavy 80-85 total seats) could do it is because AA is currently flying normally-configured 738s with 160 seats from DCA to LAX nonstop, and the DCA runway is less than 200 feet longer than LGA and DCA-LAX is almost as far as LGA-LAX. I don't know what kind of weight restrictions are imposed on those flights, but if they were severe, then I'd think that AA would fly a 757 instead.
In any event, I don't think it's a likely scenario for the next several decades. 😀