So, as I mentioned before, I'll bite the bullet and get a transformer, darn it.
I don't know how far you've gotten since March but I thought I'd share my thoughts on a similar project I just started if anyone is interested.
I started my power measuring project with a Kill-A-Watt, measuring the total power consumed through one of my breakers. It soon became a pain to go look at the display and punch the data into my tablet by hand. Since then I have purchased the wireless Kill-A-Watt and two of the sensors.
Through a mix-up from Amazon I ended up with a spare readout for free so I immediately took it apart to see if there were any ideas lurking inside. In typical P3 International (the manufacturer) style it was a pretty compact design with few clues until I get my logic analyzer out of storage. It operates on the 915 MHz ISM band and the radio chip is one of those "bonded to the PCB with a glob of epoxy over it" types. Anyway, the reason I'm posting is because of the sensor.
I don't know if anyone has looked inside the normal Kill-A-Watt but if you have you know why they put it in a plastic box. There is no isolation from the mains AC at all so you don't want to go probing around inside with it plugged in unless you are really careful. (AWOL insert cautionary gasp here. :-P) Well, the wireless one isn't any different but it's a pretty nice design and for $30 might be worth cannibalizing for a project. P3 measure the current by measuring the voltage drop across a pretty hefty piece of wire, probably just a few millivolts, and they get the voltage by a divider across LN. Since it's all inside a sealed plastic box, it's safe for consumers. I've considered cutting out the parts of the circuit I don't need and using it because it is so efficient at plugging into the wall, sampling the current/voltage and then passing the AC out to a matching socket on the face to plug your load into.
Anyone interested in discussing this? Maybe start a new thread?