Mesa 7i77 and MPG Setup

I am installing a mesa 5i25 / 7i77 setup in my vertical mill. The 7i77 supports 6 encoders and since there is no encoder on my spindle, at the most I will be using 4 of the six encoders. Since I am planning on a large number of I/Os I would rather not have to use field Inputs 16 - 19 to support my MPG. As I understand it, in my Hal file I will include a statement to tell Linuxcnc that the MPG input will be on Input 16. Can I hook my MPG into the (QA5,/QA5,QB5,/QB5,etc) terminals for encoder 5 on Terminal Block 4 and then in the Hal file map the MPG inputs to these terminals instead?

You can certainly use a spare encoder input for a MPG, one disadvantage of using a
high speed encoder input for a MPG is that they do not have a 1X mode so if you have a 100
PPR encoder with detents its hard to make it count exactly one count per detent

Notes:
1. if you use field inputs for 1 MPG you only lose 2 input pins
2. unused high speed encoder inputs can be used as 5V inputs