I am aware of the offical supported CPU list, but that list is just a guarantee not an exclusion list right?
My question is, why / what technical reason may exclude the Xeon E5-4650?
I checked the datasheet for the E5-26xx and the e5-46xx mainboords and couldnt realy see a difference that could cause incompatibility. most even run the same chipset (C602)
It all lies in the mobo microcode and whether it will support the CPU, you might try emailing supermicro support and see if they have any info on that, there are some dual socket boards out there that support the e5-4600 series.
Its normally not needed or done though, no reason to buy a 4x rated cpu when you can get a 2x, same reason you dont get the E7-4600 8x rated for a quad cpu board, the cost.