Would I recommend one? Not really. True, it would be inexpensive rainwear. But face it, the design sucks. Wind blows right through a poncho's many large openings, and they're clumsy in rough terrain. Plus every time you stick your arm out to do something, it gets soaked. Also, if it rains heavily for seven days, you're going to get wet, period, regardless of what you're wearing. Whatever the rain doesn't get, the sweat and humidity will.
But even the west coast of Vancouver Island has its dry season, from July through September. I'm not saying it won't rain, but the odds that it pours for a week are pretty low. So I see no reason why a Gore-Tex suit, along with a good rain cover for your pack, wouldn't be more than adequate. A poncho isn't going to make any difference at that point. More important will be footwear that can withstand repeated dunkings as you cross scores of streams and the occasional splash through surf. So take extra socks, several pairs of them, and perhaps some aquatic sneakers.