I'm with GG on needing the room for fleece. Like her I don't see why your idea can't work.
If there is little room for fleece, you may want to consider using preshrunk cotton, and in addition to sewing at the cuff you may also want to tack the lining in place at the thumb join to keep the lining from turning inside out when the mittens are washed.
Also, have your BF try the lining on before you sew it in to make sure it fits his hand, or better yet, trace his hand for the lining pattern as the mittens stretch, yes?
Just my 2 cents, and let us know how your combo knitting/sewing project turns out. Can't wait to hear how this works out for you...