I had Gratipay (a similar thing, I think) setup for a little while for Virtualmin. It got $0 donations. ;-)

I don't think we're gonna solve any funding issues with donations.

It doesn't hurt to try.
To paraphrase the mega successful hockey player Wayne Gretzky, "You miss 100% of the shots you don't take."
The great thing about things like opencollective is, it lets webmin/virtualmin/cloudmin receive regular monthly contributions, from backers and sponsors, in exchange for more prominence ie logo and website link, this is much more in line with what big organizations would want and need to get out of the deal. Think a big corp such as google, and educational institution such as mit labs, a large open source org such as apache foundation, eff, perl, mozilla, etc.
These days, people and orgs want and need a nice logo placement and a nice link in exchange for giving monthly.

Having solid backers and sponsors logos appear on our websites, sounds like not a bad idea, especially if they wanted to contribute on regular basis in exchange. Besides, having names of solid companies appear that use Webmin/Virtualmin is always attractive for potential users, right?

Happy to contribute a little - try putting an opencollective, gratipay or other tip jar on the Webmin login page. Every user will see it for free then, every time they use it. I've been using it for 15 years or so and I'd like it to continue ;)

Why not give it a go? It's totally free to the org. Would be awesome to get backers' individual names/github profile pics and usually well known company logos automatically added to the repo README.md by the OC markdown code. Helps backers to see they'll be in good company! And the link for a certain level of backing, is a powerful search engine boost to their website, well worth the modest cost of supporting the project.