Roadguy, I was going to recommend something like an Alivio FD-M431, but I'm seeing problems. First, you might need to mount it so low that the cage would hit the chain stay. Second, it's made for MTB triples that use longer (118-122mm) BB spindles and 135mm rear hubs. With a short spindle it won't swing close enough to the frame to drop the chain, and with a long spindle you might raise chain line issues. Third, it still might not be all that happy trying to jump the chain up a 16-18t gap, because its humps and bumps really have 12-14t gaps in mind.

Ultegra 6703 is for triples. You could knock the chain off the big ring with the outer plate, but then you'd have to do some sort of double shift to get it off the granny, which would not have enough lift to get it up to 42. Or if it did, it might push it right over the top.

Why are you doing this? No, don't answer, I really don't want to know. Truth is, there isn't a front derailleur made for the chain ring configuration you envision. They all need either a smaller gap or that intermediate step called a middle ring. Why not just get a triple? Or a dual-sport or hybrid bike with a triple? Or one of those new 2x10 MTBs?