What about an AVR Softcore, and some memory interface logic in an FPGA? DDR is cheap and easy to obtain, and i found lots of DDR memory controller cores for FPGAs. I never really knew anything about FPGAs, but your use of FPGAs in other projects including this one kinda sparked my interest to do some research, and its insane what these puppies can do.

especially in projects where I am using multiple AVRs and lots of glue logic to get work done, and every bit of that can be thrown inside an FPGA including multiple softcores (assuming the FPGA is big enough)