First off, there is a big difference between something like a fusor which is basically accelerating a beam of particles to some amount of eV that is similar to the applied voltage, and something going for a thermal distribution with same amount of eV spread out with a tail of the distribution that does most of the reactions
Fusors and polywells aren't about beams. They're about assembling a plasma object that is already hot, by compressing it during the assembly.
The fusor does this by having two concentric spherical electrodes, the inner one skeletal, with a large voltage between them. Positive ions fall inward essentially radially, accelerated by the field until they pass through the inner electrode, and fly on orbits that pass through the center of the spheres. They "pile up" as they pass through the center, thus mapping the acceleration voltage directly into compression temperature as well as high average density. (Unfortunately a small number of ions hit the inner electrode on each pass and are lost. So though it's a great fusion-neutron source breakeven isn't in the cards.)
The polywell does the same thing to electrons - with the added tweak that the inner electrode contains a set of magnet coils that get the electrons to travel in paths that mostly miss the electrode. As they orbit through the center the high average density there is effectively a third high-voltage negative electrode, producing a radial electric field between this "virtual electrode" at the center and the inner physical electrode. Positive ions fall in toward the virtual electrode (nearly neutralizing it) and again you get a high density and inward velocity, mapping the electric field into temperature.
It looks to me like the field-reversed configuration does the same sort of thing, compressing the plasma in a way that maps the electric fields (both directly applied and created by the magnetic field change) into particle acceleration during the compression, and thus into temperature. Unlike Tokamaks and similar devices, you don't "put a low-density plasma in a (magnetic) can" and then have to heat it up. You heat it by squeezing it when you initially assemble it, accelerating the particles toward each other, and that maps your compression forces into temperature - which turns a moderately high voltage into a relative particle speed that has a hysterically high number when expressed as temperature (at the same time that you're also raising the density) Hold it together long enough, don't let it interact with solid matter to cool it, and you've got the holy trinity for fusion. No ongoing heating required.
Also, you don't just easily scale up voltage past several 10s of kV, as you start reaching a lot of material limits for break down (even in vacuum), and engineering gets more difficult for 100+ kV in a small space.
- Expand the space (which also gives you more plasma volume and thus more power output at a given density), and
- Keep anything but ionized, under-control, gasses out of the working region
100+ kV is not all THAT difficult to handle in an industrial-sized volume. Air at atmospheric pressure has a breakdown of about 40,000 v/in (though this drops as pressure is lowered). A clean vacuum (except for the working plasma itself) isn't too tough either: Television picture tubes worked fine with no arc-over at acceleration voltages of about a kilovolt per diagonal inch (i.e. 25 kV for a 25" picture tube) and far more than a kV per inch inside the tube. A machine twenty feet across would have substantially lower electric field at 200 kV.
Which is not to say that there won't be issues trying to scale this. But I wouldn't expect anything insurmountable from what you've alluded to here.