Mike,
On the picture, B is neutral (that doesn't mean ground), A is 120VAC compared to B, C is 120VAC compared to B (but in the opposing phase). Therefore, A to C is 240VAC.
You needn't fear taking meter readings from a normal outlet. Don't be barefoot, standing in water, or on leather soles. If you're really worried, grab some kitchen rubber gloves. Take the AC voltage of A, B, and C each on the red lead using D as the common. Then you'll know. I suspect you'll find B doesn't show you an AC voltage. Then take the DC voltage at B. It might be only 37VDC, but it's a constant to serve as the center reference for A and C.
What I say next is conjecture, since I am clueless about Canadian wiring standards. You "should" be able to wire up for a 240VAC PSU by applying A to one blade, C to the other, and D to ground, ignoring B. The autosensing ability of the PSU "should" adjust for the difference between the DC center of the sine waves on A and C, and regardless there is an average 240VAC difference between the two.
Is there a circuit breaker or fuse on the circuit? If so, I'd recommend wiring a PSU's 12V output lines to a nice hefty transformer, and plug it in. A good PSU will go offline in an under- or over-current or an over-voltage, which I don't need to tell you. At the worst you pop the CB. Just stand back when you flip the switch.
Have you tried contacting a power department for your city/region? They may have free information on the wiring specifics and how to handle what you're doing. They like their customers alive, it makes collecting bills so much easier.