Just an idea but it might work. I came up with this solution for my friend Petunia when she had a bum thumb last winter, was wearing a brace and couldn't use gloves.

Try your regular cycling gloves, then over that a thin pair of mittens (or thick depending on what you can use) and then, a windproof shell mitten. If you look at many backcountry glove "systems" you see a laying approach just like we take with jerseys and jackets. They'll have a thin liner glove, a thicker glove or mitten, and then a windproof mitten-like shell. You may have to experiment to get the layers right and you probably want to size up to be sure nothing is too tight.

Also, I wonder. Does PI make a straight mitten? I know they made an uninsulated windproof glove last year.