Just what species is the "pasture grass"? Is it tall fescue? Is there any chance you could spray said grass before the Bermuda greened up? Then it would die and the Bermuda would fill in to some degree--and when it was warm--say when the Bermuda had been mowed--it should be warm enough for Bermuda seed. That would be about daily highs of 80 to 85 I suspect, (but I am in the north--local experts are best).