Can't you just add in Wikipedia-style donation request popup/banners for everyone in general? I reckon plenty of people will be willing to spare some change even multiple times when they log on, just like they do with Wikipedia.
If anything Wikipedia as an example of how unsuccessful a voluntary donation model is. 0.2% of people who use wikipedia donate to it. Unless you are doing tens of millions of unique views a month, it's probably not gonna be sustainable. People really hate paying for things. Even things they use and love.
Endless telethons for public television in America annoy 100 people for every one that donates. Despite that you see ads on public TV for the Archer Daniels Midland corporation all the time even though, allegedly, there is no advertising on public TV.