Treat it as a commercial professional project with a special deal for open source projects.
Donation buttons don't work. Donation campaigns work a lot better - see this Mixxx post: http://mixxxblog.blogspot.com/2010/12/build-server-fundraiser-success-thank.html
"We've raised more money in 3 months
than we've received in donations since
Mixxx was started in 2003, and for
that, we'd like to thank the 43 Mixxx
fans who generously donated to our
cause, and our broader community for
helping spread the message."
Now look at the amount of money they raised in those three months... enough to buy one PC. Are you going to have the same high traffic as Mixxx?
Treating it as a commercial project won't alienate users. You just are likely to have fewer people trying it out out of curiosity, a filter for those who see a clear need for it. No bad thing.
If you're not prepared to invest time in a protection/limited-trial mechanism, then it is a must that you have a nice GUI and nice screenshots so that people can see it is not a toy, and be seduced into paying for it up front.
Once money is coming in, question whether you really want to do that special deal for open source developers. As fanatic coders, they are rather likely to want to create their own version based on your ideas. You'll be hastening that competition. What I am saying probably sounds very mercenary, but if you need a direct income from your product, you need to put that before your other goals.