That would work, assuming the lowpass filter held
in there all the way up to 20 GHz.
be tempted to put some round ferrite toroids along
the outside of the coax cable--just to keep any
standing waves down. If there is a big standing
wave, then you could get leakage thru the ground
braid at the E field maximum points. The ferrite
toroids would de-Q the outside braid so the standing
wave would be small.
If you still had leakage,
and needed to add another lowpass filter outside
of the chamber, that is where it gets dicey. Since
the lowpass filters are totally reflective, if you
have two of them at either ends of the cable, you
can make a resonant circuit out of the whole mess
and lose your rejection. A standard fix is to add
a few dB of attenuator pad at the outside LPF, but
that would screw up your passing of DC.