Well I've never done a lined mat before and hadn't had a real desire to learn. I also had never had a customer bring it up before.
Guess what? Just had an existing customer come in with a print that she wanted framed to compliment another french lined mat. All of the advice and examples I have seen from other framers both here / FB pages and the G gives me confidence that I can do it.
Now just have to order some supplies and practice, practice, practice.