The closest you can get is to select "Cheapest Economy Fare" and then select "Upgrade Elligible" with the particular fare basis block you want.

So if I want an upgradeable B fare SEA-LHR, I select "Cheapest Economy Fare" and "Upgrade Elligible with Miles Y/B" and I get a $1400 B fare. If I select "Cheapest Refundable" and no Upgrade Elligibility, it spits me out a $2400 Y fare.