I like the Boston Acoustics VR-MX as they're attractive, are voiced for boundary support (unlike most!), and can be set up as both direct and ambient-firing depending upon your room's geometry and balance. Good Luck.
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In addition to Subaruguru's good suggestion, I would recommend you look at M&K's line of tripole speakers. I have a pair of the SS-150 THX tripole surrounds, and find they blend very seamlessly with my Vandersteen main and center speakers. It is sometimes difficult to get a good match between planar/electrostatic speakers and dynamic speakers, and that is one reason the tripole design might work well.
M&K has several new tripole models that sell for as little as $400, with prices up to around $2k. Check out their web page listing surround speakers at the following link:
It seems like you already have an idea on ceiling mounting the speakers. You may also want to consider whether the room is enclosed or open. For an open area, direct radiating speakers may be preferable over dipoles and you may wish to have speakers similar to your front Martin Logans. Magnepans makes a line which installs on the walls with a bracket and which can be folded out of the way when not in use. On the other hand, for an enclosed room, consider dipoles such as the Paradigm ADP. I believe they come in white and can be fairly unobtrusive.