Elizabeth May has declared that the Green Party will sit out a Labrador byelection and is calling for the NDP to do the same in order to help Liberal candidate Todd Russell, who lost to the departing Pena Penashue of the Conservatives by 79 votes in 2011.
Liberal leadership hopeful Joyce Murray is taking credit for the call to cooperation, and if successful could raise her profile in a leadership race that most expect Justin Trudeau to win based on his expert haircut and accomplished last name. Murray has been the only candidate to endorse proportional representation, despite the fact that the Liberals use instant run-off voting to elect their own leader. The Green Party and the NDP platforms both support proportional representation.
Should the gambit work, it could demonstrate the efficacy of tripartisan efforts and create a germ of hope for electoral reform in Canada. Alternatively, Canadians can keep allowing for a minority of voters to elect a majority of MP’s. In 2011, the Tories won 53.9% of seats in parliament with 39..6% of the popular vote.
Gods & Services 
Although I’m inclined to agree with you on applauding tri-partisan efforts at toppling the Conservatives, I can’t say that I like the Liberals much more than the Cons. (Though I acknowledge that only the Liberals and Conservatives are, to date, capable of forming governments at the federal level.) Though I often agree with Liberal policies in theory, I cannot tolerate their record of corruption, and can only hope that their recent electoral nadir somehow shook them out of those habits.
We’ve talked about proportional rep before. I personally would be happier with an instant run-off system like the parties use internally, than with straight-up proportional. (For me, the main goal, regardless of the “how” of the election, is to get preferential ballots into the system, as we might get in Toronto, if we are truly lucky.)
I worry that the Liberals would not be inclined to promote voting reform and so I am concerned that Canada’s legitimate opposition parties (as opposed to the Marxist-Leninists et al.) might throw their lot in with the Liberals in order to topple the Conservatives. If this somehow worked (and I doubt it would, but you never know) I think it’s more likely the Libs betray the NDP and Greens than actually stick to any kind of systemic reform commitments they may or may not be forced to make as some kind of tri-partisan deal.
The Conservatives won a slim majority, with only 11 extra seats in 2011, after a series of minority governments. I suspect the NDP and the Liberals could find enough ridings in this country where one or the other is clearly stronger, but not strong enough to defeat the Conservative candidate in a three-way race. With the three leading parties each hovering around the 30% mark in polls, there seems to be a narrow window of opportunity for the NDP and Liberals (if led by Murray) to cooperate without either having too much of an upper hand over the other to create the opportunity for betrayal you speak of. The NDP did very well in the last election, particularly in Quebec, so if a deal was made the Liberals would have a hard time arguing that more of their candidates should get to run unchallenged from the left than vice versa. The participation of the Greens would be of marginal importance, but Elizabeth May has proven to be a very competent parliamentarian, so her involvement would be helpful.
I have a preference for mixed member proportional representation, but instant-run off voting would still be a huge improvement over the current system. In any event, electoral reform is badly needed.