How to get the transfer characteristics of power strip lines?
Look for something called a Line Impedance Stabilisation Network, or LISN. You'll find a circuit diagram for it in MIL-STD-461.

It's essentially a diplexer, a high pass filter for signals above a few kHz, and a low pass for the mains current at 50/60 Hz.

The Stabilisation part of it is that it isolates the circuit under test from the (presumably very low, and unknown) impedance of the mains supply.

In your case you'd turn it around and use it to isolate the load, and feed the RF into the mains.

I don't think the LISN goes as high as 500 MHz, but up there the mains impedance is irrelevant. You can just disconnect the power and test it directly with your equipment. To be thorough, try a few different power extension leads on the power strip, to get an idea of the range of values you can expect in practice.