Dedicated DSP for delayed replay
Hard to believe, that you actually looked. You probably don't even need a DSP for that, a normal microcontroller should be sufficient.

For example, the STM32F4-Discovery board comes with an ARM Cortex-M3 that has several ADC channels that can do 2.4MSps each and up to 7.2MSps in interleaved mode. There's a DAC for signal output as well. But since your signal is digital anyways, you can probably sample with just a normal digital input pin. If 1bit per sample is enough, 250ms of a 2MHz sampled signal require 62500bytes which fits into the memory of the ARM quite well.