I've been wondering recently how viable it would be to implement an FPGA-within-an-FPGA:

  * Create a model of a simple, open FPGA.
  * Create tools to support the open FPGA.
  * Synthesize the FPGA for an existing proprietary FPGA.
  * Install bitstream in SPI NOR.
  * Ignore the proprietary FPGA from now on - work with the open FPGA that's inside it.

Yes, it would be absurdly inefficient. However, you can buy pretty huge FPGAs for very little these days. If the ratio of host:target LUTs isn't too bad, and you can find constructs which synthesize efficiently, it might yield something usable.

Perhaps it could be a way of bootstrapping an "open" FPGA effort.