This is a good idea but may I suggest using a Gentoo base instead of BSD? BSD is really great and I believe mac's OS X uses shell version of BSD but I don't know if Mint's developers would want to build Mint version on something that isn't Linux that they've been working with. In fact, I believe Clem in one of his introductions mentions that he was providing a lot of help in Gentoo community which gave him ideas on how to shape Mint. So there might be some experience right there that might help jump start Gentoo branch.
As for myself, I am a big fan of Gentoo. Our company uses one Gentoo-based dedicated server and one VPS. Gentoo is a great and stable distribution, is excellent as a server and works on the same principle like LMDE, ie, rolling distro idea as it becomes version-less after install, so there will be little difference for people to get used to as it would work like already existing LMDE