I bought an MEC union suit in the quest for a one-piece base layer. The zippers are too large and they will grind your taint to a bloody mess. The suit is made of low quality materials. The metal zipper is in direct contact with your nether regions, where it conducts cold remarkably well. In short it is useless.
I bought a Gamma Salopette from Arcteryx and have given up finding a piece base layer to go under it. The Arceryx wool base layer bottoms are great, but no suit is available. The OR quadratic suit does not fit my body type. My torso is too long. If it fits you, some are available on the web. I think I sam some at Moose Jaw mountaineering. However, for the price you could have a custom garment made.