There was a flaw in the USTA system a few years ago that would reveal the exact DNTRP number. Basically, as I recall, if you appealed your rating on the USTA site, it would send you an e-mail. At that point, if you took that e-mail and displayed the source coding/header information, the DNTRP number was contained within this data. After learning about the flaw, I tried it myself and it was true. I had been bumped up that year, and my appeal was denied. It turned out that I was 0.01 points away from the threshold to be automatically granted the appeal. (In other words, if you were within the 0.01 and 0.05 range from your former rating, they would let you go back down... but I think I was a 0.06.)
Anyway, this is all a moot point now because someone informed the USTA about the hole in their system and it was fixed. I'm sure that some enterprising person can hack their system if properly motivated, but the easiest method might be through human subterfuge. I had thought that USTA League Section Coordinators had access to this information, but that might also be outdated information.