You have other options as well using modbus boards both of these hook up through the serial modbus in mach3:
1). Peter Holman sells one called the Mod IO, it has 3 Analog in and outs, 8 digital in, and 8 out Plus MPG.
2). Aurturo of CNC4PC has a M13/M1 that has 4 analog in, 4 analog out, 28 digital inputs, 28 digital outputs.
There are other options that you have already discussed as well.
scott