Here is my pinhead take on the problem: 1) keep it simple and 2) chamber quartets were not written to be heard up close and personal. I would go for the AB omnis or ORTF. It will keep you from introducing phasing issues and keep the sound the way the audience would hear it normally.
Williams does describe a three card array which would be just what you are looking for, however. I have tried it and find it works well and is somewhat similar in what it "hears" to MS.
Stupid M/S question...
Please post your results.