FYI, the macropump that I used is;

SetModOutput=(81,GetUserDro(4))

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.