You'll need to call, but you can use ITA matrix to price things out. Put 'DL+ /f bc=m' (without quotes) into the advanced routing codes for the direction you want the M fare and leave the class of service set to cheapest available. It will then find LUT (or whatever is out there) for the unconstrained direction. If calling the US medallion line, I'd also set the point of sale to US, as fares might differ a bit if not set that way.