Here's what I did (mostly my own design, with some input from Jeff):
I installed a new double-gang breaker, which gave me access to both phases. I ran a 12/3 line from the new breakers to a junction box that I mounted right near the breaker box.
In that box, I installed a regular duplex outlet, but I split-wired it (top to one phase, bottom to the other). I also installed a 240V socket, connected to both "HOT" lines.
Per Jeff's recommendation, I wired a short length of wire to the XTB-IIR, and terminated that with a matching 240V plug. That gave me an easy way to disconnect the XTB-IIR for testing.
Since Home Depot didn't have a cover plate that matched what I needed (duplex outlet on one side, single round outlet on the other), I made my own by cutting a large round hole in a Duplex outlet/single toggle switch cover plate. the screw holes line up for that.
I now have a dedicated "test" station, adjacent to the breaker box, where I can plug in a signal meter (if I had one), or where my dad (an EE) can plug in the fancy setup he made with his oscilloscope (using Ido Bartana's plans), to look at noise and signal strength.
I can look at both phases separately, with and without the XTB-IIR connected. I can plug a transmitter (CM15A or RR501) in at one of those outlets (I might need a short extension cord for clearance), and look at signal propagation.
One other benefit of this test station is that the only thing plugged into those two circuits are the XTB-IIR, the CM15A (plugged into the XTB-IIR), and any test equipment. I can shut down EVERYTHING else in the house if I need to, to try and track down noise. Before, I could shut down most things, but I needed to leave the circuit with the CM15A on, and unplug everything else from it.
Since I installed the XTB-IIR to try and solve an issue I was having (which it actually couldn't solve - turns out I just needed to buy my neighbor a new $7 CFL bulb), I used the test station a whole lot at the beginning. However, it doesn't get much use these days (well, the XTB-IIR outlet does, the other two don't).
One of my earlier thoughts with this whole setup (before I made the decision to get the XTB-IIR), was to remove the capacitor I have that is bridging the two phases in my A/C compressor disconnect box, and wire it to a matching 220V plug, so I could plug it in close to the breaker box. It sits (Jeff said I didn't need to remove it when I installed the XTB-IIR) inside the A/C disconnect box (because that was a convenient place to mount it, that had access to 220V, and I could shut the entire box down at the breaker to work on it much more safely), which is nearly 50 feet from the breaker box. Considering signals had to travel through an extra 100ft of wire (50 there, and 50 back to the breaker box) when crossing between the two phases, I thought perhaps moving the capacitor would fix my issue (it wouldn't have).
Sorry for the long (and not all that well organized) post.