I have bose active noise cancellation on my car stereo, so its certainly feasible for a PC. However, it isn't as effective as it is on the headphones and only mildly reduces the low frequency noises - it doesn't completely cancel them.
In terms of 50Hz, which has a wavelength about 22.5ft, or 6.9m, the distance between the heads of passengers would be on the order of 1/4 wavelength. I am unfamiliar with the nature of road noise, but it would be very difficult to use a few point sources to actively dampen a wide frequency range when interference patterns come into play.
Is this an after market device? To calibrate I would think you would need to place a mic roughly where each passenger's head will be and build a table of phase offsets for the output of each speaker. Otherwise I very much doubt it is doing much, unless I am overlooking something.
Have you tested turning it on and off? What kind of noises does it work on? Is is more in the 20Hz range, or does it go higher?
Anyway, I guess if you know roughly where any ears are going to be (within ~1/16 wavelength or better), have a sufficiently complex output system (1 point speaker per point of interest or more), and have a fairly acoustically static room (no moving a speaker, large furniture, etc), you could set up a working system... But it would be pretty time intensive to set the microphone in the correct area and get phase offsets for all frequencies of interest... Oh, and if you are talking about 5 people in the room, and 50Hz sound you may end up needing 5 subs depending on how close they are sitting. You might be able to arrange your seating to minimize speaker requirements though :)
Anyone know if I am overlooking something? Assuming point sources and individual points of interest I just don't see how you get a generalized method for the interference without an equal number of speakers and points of interest, and proper calibration. Are there approximations/simplifications that can be made?