I ran into an excellent, thorough explanation of how ADSL line filters work today at epanorama. Contents include an increasingly in-depth description of the theoretical and practical purpose of such devices, photographs of disassembled filters, and what really caught my attention: a circuit diagram.
I work for a telco, will try to read this tomorrow on paid time.
Please don’t attribute any of the formatting errors or typos to me; all I did was trip over it while trying to confirm what the cut-off point typically is for filtering (couldn’t filter lower than 4kHz, would have to filter at least as low at 25kHz).