I think short event or campaign with push for donations with a pop up that you actually can dismiss. An ad like banner. The biggest problem would be community organization as Lemmy isn’t only decentralized horizontally but also vertically. Different front ends, different apps different instances. Most of them wouldn’t want to implement an ad that wouldn’t benefit them directly. They also have costs with running their piece of lemmy. So some cut for them should be included.
I think a dedicated trustworthy person should be responsible for organizing this campaign as developer time is best spent elsewhere.