I haven't used it with a Pi, but I have used the serial interface and a terminal program. The last page of the manual has the commands:
http://www.elecraft.com/manual/W1_Power_Meter_Rev_D.pdfYou could set up a USB-RS232 serial device, or utilize the console port on the Pi (run raspi-config and change the console port setting). Then just telnet/ssh into your Pi and run a terminal program like minicom to use the power meter. Not the most user friendly way to use the meter, but it will get you started.
Depending on what you're wanting to test, you might have a problem shielding the W1 enough to keep stray RF out. You might have to put a dummy load on the output or run extremely low power, or energy from the antenna will get into the current transformers and give you false readings. And of course RF could screw with the Pi or the PIC on the W1 as well.