Hmmm - I wonder ... presumably the BPL couplers are capacitors coupling
appropriate RF from one leg of the power system to the next. Would they
also couple spuriously-generated RF at similar frequencies?

Absolutely. The couplers I have seen inductively couple the BPL signal from the
low-voltage wiring onto a single phase of the medium-voltage wiring. One
manufacturer has provided an estimate that they have about 6 dB of loss. Right
now, you and perhaps 3-4 neighbors share a common electrical-wiring noise pool.
Their noise goes out their AC wiring and into yours. These couplers will
create community-wide noise pools, taking all electrical noise -- BPL or other
-- from house wiring, where it is radiated inefficiently, to overhead wiring.
You guys know about transmission lines and antennas -- do the math.

Will the entire BPL scheme spread all these other
spurious emissions far and wide?

Yes, it will.

And, by the way, since aircraft use AM, do they receive hash when they
fly over BPL lines? Are our brethren in aviation potentially useful
allies in this war?

http://www.arrl.org/news/stories/2003/11/14/101/?nc=1

73,
Ed Hare, W1RFI