Another idea.
Take a Cortex M3/M4, add a few MB of SDRAM on its EBI port, let it run a basic USB host.
You then plug the flash drive into the unit, cortex copies files from USB to SDRAM, and voila, deterministic clocking at any speed you want.
Hell of a lot less dev time / expense than an FPGA. A decent 160Mhz+ M4 will probably do 20 SW UARTS without bitching to much.