Thanks Ben - that's extremely helpful.
Neon ionization: The temperature we're taking for this plasma is of order 6000K, which corresponds to about .5eV. Because neon is a noble gas, its ionization energy is high - around 20eV - and its first excited state is a large fraction of that. That means very little of the neon in the Mozina plasma will be ionized, because there is a huge Boltzmann suppression e-20eV/.5eV~10-17.5. So nearly all the neon will be neutral and in its ground state.
Photon energy: The wavelength of interest is 171A, or 72eV of energy per photon. That's far above the ionization energy for neon (20eV). Therefore, as Ben says, the photons in this band can scatter by ionizing neon atoms. The cross section for that in the 171A range is a bit larger than 10^-18 cm^2. Since I'm trying to systematically underestimate the opacity, I'll use 10^-18.
Bound-free opacity: We now have the information necessary to compute one of the contributions to opacity, the one coming from ionizing neutral neon. The opacity is κ=nσ/ρ, where n is the number density (of neutral neon in this case), σ is the cross section for scattering with the photon, and ρ is the mass density. The number density is the mass density 10^-7 g/cm^3 divided by the mass per neon atom (3.5E-23g), which gives n=3*10^15 neon atoms/cm^3. Multiplying both sides by ρ, we have κρ=1/(3.5*10^2 cm).
Attenuation of 171A radiation: The intensity after passing through some thickness of material is I(x)=I0e-xρκ, where I0 is the intensity at the source or when the radiation first enters the plasma and x is the thickness. So what this tells us is that for every 350cm=3.5m of plasma our 171A radiation passes through, its intensity is attenuated by a factor of e=2.71. To give some sense of what that means, after propagating through 1km of plasma, the intensity will be reduced by a factor of about e^(-1000/3.5)=10^-124.
Necessary intensity of source for visibility: Therefore, for one photon to make it through a 1km thickness of Mozina plasma, we'd need about 10^124 photons to be emitted by the source. Each photon carries 10^-17J of energy. So that's 10^107J of energy emitted by the source, which is vastly more energy than there is in the entire observable universe. In other words it is impossible for even one photon of 171A radiation to propagate through 1km of the Mozina plasma, no matter what the source.
I may well have made an error somewhere in there, so I invite critiques. Thanks to all - I learned from this.