I've hit into the same problem and this thread was a good hint!
But I'd like to do my bit:
maybe I did something wrong but useing untapped coordinate didn't worked in my case.
Contrary, useing any spare variable works fine. So my version of Macropump.m1s is
SetModOutput(80,GetVar(1)*409.5)
SetModOutput(81,GetVar(2)*409.5)
and G-code should contain something like
#1=5.0 #2=10.0
to setup 5V output on analog output 0 and 10V on output 1.