Adding 3rd MPG to 7i84

Is there any 'elegant' solution to adding more than two MPG inputs to a 7i84? 

I'm already using the two built in encoder/MPG inputs, but would like to interface one more 5v MPG. The only solution I've come up with so far is a wcomp for the MPG outputs, and tying these outputs to encoder.N.phase-A/B. 

I think the limit is the sserial packet size but I guess a variant could be made that drops
some data (or uses the new expanded sserialb)

Are you saying that it is possible to add additional encoders to the 7i84 by modifying the firmware? I assumed that the encoder limit was hardware based, not software.

I am only using 4 outputs, so I could lose some outputs, if that meant I could pick up an additional encoder.

The encoder is for a 20ppr MPG knob for spindle speed override.

The 7i84 is connected to a 7i77, which does have a spare encoder input. I could use that input, but the knob is already wired to the 7i84 and I don't want to add additional wires from the control cabinet to the main enclosure.

I have 24v for the 7i84's field power supply, but the MPG is 5v. The only solution I've come up with is connecting the MPG to an analog input on the 7i84, and then in HAL passing the signal to a WCOMP and then to the software encoder. I was just wondering if there was a more elegant solution.

There is 7I84 firmware (7I84ET.BIN) that allows you to set the external thresholds
of the A and B inputs from LinuxCNC, that might simplify the software encoder a bit