I did this about 5 years ago, so I don't know if the technology is applicable anymore. I was configuring a Xilinx Spartan II FPGA. That device supported JTAG and/or Slave Serial configuration and I think my 8051 firmware had to support both (if memory serves). My meagre notes in the header follow:

/**************************************************************************
* Description:
*
* This file implements the configuration of the Xilinx Spartan II FPGA by
* either JTAG or Slave Serial means.  JTAG being somewhat more complicated,
* takes about 3x as long as Slave Serial to perform the configuration.
*
* The data used for the configuration is read from the "JTAG File" located 
* in the serial NV/Flash IC (connected to the CPU via SPI).
*
* The JTAG file (assuming that slave serial is not the method of choice)
* contains a series of XSVF records.  XSVF is a compressed (binary) version
* of an SVF file.  SVF files are created by the JtagProgrammer Win32 app.
* The files are then converted to XVF (XSVF) using CONVERT.BAT.  The file
* contains a series of instructions for negotiating through the JTAG state
* machine and reading/writing data. Typically, the XSVF file will contain 
* instructions near the beggining for verifying the part type and also
* there should be instructions near the end for starting up the device and
* checking the DONE bit.
**************************************************************************/

If this looks interesting to you I can try and locate further information. Perhaps contact me by email and I'll dig further:

GB
grant@genovation.com