foam hot wire CNC YZ VW planes

Why can't you just re-name the axes?

By which I mean, why do your axes have to be YZVW?

My understanding was that hotwire CAM used XY UV.

If it is possible to persuade the CAM to use XY UV then things will be much easier. This might be as simple as setting the machining orientation.

I assume that you have found the built-in FOAM option in Axis?

Have a look in the sample configs for sim/axis/foam to see the special Axis foam-cutter display.

As far as I can see, nothing in the documentation even mentions the option.

Yes, the "sim" configurations are simulators, designed to show what a machine would look like.
(Some even include a 3D model of the machine, look under sim/axis/vismach for those examples).

You are right, stepconf can't make a foam cutter config.
However, you can make an XYZA configuration and hand-edit the HAL and iNI files to make it into a FOAM configuration.
If you look at the sample foam configuration you can probably work out the differences.

The main changes are, in the INI file [DISPLAY] section
I tried to set up an XZUW machine, and it can be done, but Axis doesn't know how to display it, so I suggest not doing it:

Are you sure you changed the correct ini file, and that you put the new lines in the correct section?

If you change the "MACHINE" name in the INI then you will see that in the header bar of the Axis window (<filename>.ngc Axis <version> on <machine name>). That is one way to be sure that you are opening the config file that you think you are opening.

It might be easier to be sure by starting LinuxCNC from the CNC menu.