After pumping on the clogged level sensing line for a short period this morning, I isolated the line by closing the in-line ball valve that is part of the adapting hardware.  Next, I removed the vacuum line and connected the actively flowing/purging UHP N2 line in its place (10 psi @ 1 lpm).  The clogged line then can be back-filled with dry N2.  Once back-filled, the adapting hardware is removed and the (slightly positive pressure) line then equalizes to room pressure.  The open line then gets connected to its nominal plumbing (local mechanical magnahelic differential pressure gauge in parallel with CDS pressure transducers).  At this point, the pattern is for a buildup of pressure until both the mechanical pressure gauge and CDS transducers "rail" at 100%  Keep in mind when looking at the data that the rate of this pressure build-up has been sporadic due to my inconsistent making up of the plumbing compression fittings - sometimes by hand, sometimes with wrenches - it turns out that even small leaks into the room confuse the issue.  Regardless something WONDERFUL is happening in all instances.  That is, gas is building up above room pressure after the equalized line gets connected to the "closed" non-leaking plumbing.  Each time the plumbing fitting is loosened, I can hear and feel a significant amount of built up gas being released into the room.  The only explanation the I can come up with is that LN2 is getting past the plug and vaporizing in the warm section of the sensing tube or the plug is comprised of a high vapor pressure material and is "inching" up the sensing line and getting warmed.  

I was only able to try two iterations today and am leaving the line connected to the UHP N2 (10 psi @ 0.5 LPM) over night.  Will try some more tomorrow.