External DIO
It would be worth considering to support at least 5V for input range of the external input. 3.3V LVTTL is, IMHO, quite limiting from the external world.
An idea would be to reuse the trigger circuit of the FMC ADC 100M 14B 4CHA.
Consider also using a double stacked lemo (LEMO_EPY.00.250.NTNY component) to separate input and output to ease the implementation of above mentioned suggestions.