Actually, 2.10.720 is available for Mellanox branded ConnectX-2 cards. It's part of the Windows Server 2012 OFED package. During the install, if you have a supported card it will flash the firmware to the latest version. Mellanox does not have the firmware available as a separate download.
Mellanox says it's up to the OEM to provide the updated firmware for their designed cards which is understandable as there are Dell, HP and IBM and who knows what else proprietary designed cards. If I were Mellanox's shoes, I wouldn't want to be on the hook for cards not designed by my own company.
With that said, I have contacted Dell and brought the issue up. It turns out the C6100 has not gone thru any kind of Windows 2012 testing and the engineers aren't even aware of the firmware issue regarding the mezzanine card. They are looking into it but I could see it going both ways as far as support/providing a new firmware. Hopefully I will hear something this coming week.
In the meantime, I did acquire an MHQH29C-XTR which is Mellanox's own PCIe card that the Dell mezzanine card is based off of. Just about everything is identical hardware wise, with the only difference is the part number and PSID burned into the EEPROM. I was able to extract the 2.10.720 firmware out of the Server 2012 OFED package and posted up instructions on servethehome.com forums where someone was able to customize it, and successfully flash a Dell mezzanine card to 2.10.720. RDMA shows as enabled now and we're just waiting for some further tests to see if it actually works. Follow the thread here: Dell C6100 and Windows Server 2012 no RDMA.