Safe way to interface directly with HomePlug / Powerline
Intersting.  I'm assuming you want to see beyond (further into) what you'd get out of a vendor's silicon solution.  If one knew the specific protocol at work, then I'd go looking for the datasheets of that silicon vendor's chips, &amp; in there (or in related AppNotes) you'd likely see reference designs that'll show you what kind of mains line interface is recommended (voltage transformer, current transformer, filtering, AC-coupling, etc).

And for (a) the sake to saving you from constructing an interface yourself AND (b) to leverage the relative safety of the vendor's existing design rather than having to get a bunch of mains safety stuff right yourself, perhaps hack into a vendor's product to get at the analog signals before they disappear into their silicon.

Naturally all the warnings and safety recommendations about working with deadly voltages applies here.  Highly recommended is to seek out someone who knows how to handle mains safely to get get you started.  And put an Earth Leakage Circuit Breaker between your test rig and the wall socket. etc etc etc.