Would you see a non-denominational ceremony as a cop out?

It is never a cop-out to stand by your convictions. A cop-out would entail having the wedding at her church just to shut her up. My husband's family and mine are both Catholic, though neither my husband or I are. We chose to have a beach wedding, which naturally caused some upset. In time, everyone got over it, and it was an incredible wedding. You just need to be ertive and your fiance needs to explain to his mother that this is the plan, and it is not open to negotiation. We placated my MIL by allowing her friend (and priest) to say a blessing over us at a family gathering a few months later. I saw that as something for her, rather than for us, but I really like her, and it made her feel better, so I didn't mind going along. Maybe you could offer her something like that, if you're inclined to do so.