I came across a blog entry yesterday from Derek Wall (http://another-green-world.blogspot.com). He's a high profile member of the Green Party of England and Wales and a member of a group within the party called Green Left. The article is called Green Left call for support for John McDonnell.
It's a interesting article essentially calling on disenfranchised Labour MP's to stand as independant candidates at the next General Election and comes with an assurance that those that MP's that did this and "stood for meaningful action to combat climate change" would not have a Green Party candidate stand against them.
Whilst an interesting thought, why not take this idea a step further and get the Green Party to send an open letter, say via the newspapers, to all Labour MP's (and Liberal Democrat and Conservative MP's come to that) who stand for, or have any kind of concern over environmental issues to join the Green Party and stand as a Green Party candidate at the next General Election. One of the problems the Green Party has is getting it's message out to the public through the media (particularly the TV networks). With something like this, it would be very hard for the media to ignore.