Chris research:
I tested the G5.1 option in WinCNC and it works. It pops up a dialog box with a text entry field. The value is stored in one of 10 registers, numbered GR0-GR9. That value can then be used in gcode.
Here's the syntax: (r is the memory register, m is the message)
g5.1 r0 m"please enter a distance for X travel"
Enter a number in the box, then to move the X-axis that distance, enter
g1 x{GR0}*note the curly braces around the variable