When purchasing Guild Wars 2, I was fully aware that it was a Windows-only game, and that’s fine. Also, I don’t expect ANet to just do something for free out of the goodness of their hearts (though it would be nice).
So, how about something like a Kickstarter program for a Linux client? I would pay an additional $20-$30 on top of what I have already paid for Guild Wars 2 to fund development of a native Linux client, and I’m sure there are many more out there who would do this as well. Do some math, find out how much it would cost to pay a developer to make and maintain a Linux client, and set that as a kickstarter goal (or something reasonable).