The easiest thing I think is to ask wordpress.org if it’s possible that they host your plugin with GPL v3+ compatibility. Probably they say yes. Give it a try, the discussion was very new back in the end of January so points of views might be a bit more settled these days.
IMHO there is not really something to loose if they allow GPL v3+ plugins in the repository.
In case they find code that is cool for core, they can ask. I mean, who would not relicense code then if it’s possible. It’s nothing as static as it’s often said.
The next thing you can do is to ask the excanvas project if you can use their code under MIT as well. Probably they do allow you that. Ask them. Probably that’s already possible, because flot ships under MIT and it incorporates the excanvas code in it’s package. But you should always ask on your own.