Switching mode power supply for Power Line Communication
First, just assume the power line is noisy and design your protocol accordingly.  A competent switching power supply by the device shouldn't make this substantially worse because you already assume there are switching power supplies hanging off the power line in various places.

Second, a line filter doesn't have to apply both to the power supply and the signal connection to the power line.  This really should have been obvious.  Put the line filter between the power line and whatever power supply you use, but tap the line for the purposes of the signal connection on the line side of the filter.