Seems like if you sprayed right before the planting (like the day before) the roundup would be dry and not affect the nonsprouted seed. Then the corn would get a jump on the other weeds and might shade out some of them. For $10 it would probably be worth the attempt, versus $250 for a bag of corn... unless it is less than that now. That was what it was the last time I planted it.

And then spot spray a little of it after it comes up, if it dies then let it go and hope the weeds are at a minimum. If the roundup does not kill it then spray the whole field.