Unlimited custom outputs?

with a custom component you could also use an analog digital input. Say you had input numbers 10,20,30.... 
you could use ranges < 15, 15-24, 25-34 etc to decode you input number for a near infinite number of values on one pin and change with m67-m68
eg
M68 E0 Q30   (set relay #30 on)
M68 E0 Q31   (set relay #30 off)

With some creative hal programming, you could probably use arrays so the code would be very compact.

I would like to activate them from g-code.

Why 7i73? I used a Mesa Card Configurator and it showed multiple 7i73's but you are right, the 7i72's would be better.

I would like to use M codes. I had not considered using variables to increase the number of outputs. Is there an example of that available on the forum?

I had posted one of my configs before that uses this for M101, M102, M111, and M112. The commands are for opening and closing a series of fixturing clamps (6 air solenoids) on each side of the machine. M101 and M111 are for closing the work holding clamps (left and right side respectively) in a sequence to help correctly position and hold the parts to be milled.  They use both a P and Q variables (P sets the clamping value and Q an unclamping one.) The clamping sequence can also be changed depending on another parameter variable for different parts. M102 and M112 are primarily for opening the clamps to release the parts and only use the P variable, but depending on the P variable used, can also open or close any array of the clamps immediately without any sequence. All of these M-codes write the appropriate integer value to the appropriate Hal signal name (left-clamp-fl or right-clamp-fl) that then feed the Hal pins that make the magic happen.

Here is a link to the config.
forum.linuxcnc.org/media/kunena/attachments/3190/TwinCNC.zip