Connecting high speed DDR ram to an FPGA is not a big deal.
I see no reason a DSP based FPGA with DDR ram attached could not run SCRYPT.
The FPGA by itself would not be effective.
I was under the impression that the kind of memory latency necessary (extremely low) is only possible with the most expensive types of DDR chips.
I'm not sure if the economics of the scrypt mining business has changed, but I do believe the math was done on this a year ago and it was a total waste of investment.