What might be useful is an in-line filter, not an extension block but just an adaptor that goes between plug and socket.
This is just me musing over how hard it'd be to filter mains wall sockets - given the varying depths of back box/walls/plaster(board)/etc I'm not convinced that it'd be simple, so the thought occurs that an adaptor in between plug and wall socket might be a better solution. Give it massive insertion loss (70dB+) from 10kHz to 50MHz and it would cover all ADSL/VDSL frequency bands.
If FTTC self-install happens then I can see a fair bit of work along these lines for enterprising sorts and having a filter to quickly eliminate/indentify conducted noise problems would be handy to say the least.
Of course it would kill powerline networking stone-dead but that's no bad thing - I can see them causing widespread issues on the higher bit-rate (17a/30a) VDSL2 offerings sooner or later anyway.
Hmm something to think about now summer has arrived and its raining again