Not possible unfortunately. You can use a tool like matrix1.itasoftware.com to price out the domestic legs at the lowest fare and then the overwater at M to see what it should price. But you would need to call to book. In some cases, the M fare domestically is roughly the same as the total M fare so it may not be worth calling.
I asked the GM desk about this, and you can have them book different clases of service. I was looking at M on my long-hauls so I could use Skypesos for upgrades...unfortunately, even with no seats showing occupied in BE, I couldn't upgrade. Go figure.
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2010 GM Upgrades 13/13
2011 GM Upgrades 0/2 (Don't leave DC on a Friday afternoon to ATL!)