I don't think it's supported for Spartan-6 though.
When I looked at the encryption format for the virtex 5 series it was quicker to reverse engineer it than it was to find someone within Xilinx that could explain it (just basic bit/byte order, what's covered by encryption, etc).
At the end of the day, FPGA configuration logic is just another state machine ;)
The easier thing to do would be to delay enumeration.