I never received any reply from linux-projects.org after more than a week. I didn't find any way to remove the overlay.
By google I only found some links to other people who tried and gave up:
https://www.reddit.com/r/raspberry_pi/c ... watermark/
https://raspberrypi.stackexchange.com/q ... -watermark
It's a shame, UV4L can do a lot, and would make things much easier for me. I'd like to use it in some art installation projects I'm working on. But having the "software by linux-projects.org" banner on the screen makes it completely useless for my purposes. Now I'm stuck trying to use Chromium with Node.js servers, but it's a struggle because I don't know much about it, and so far it doesn't work very well.
Even if I could get three free unlock codes, that's no good, I need something I can rely on and use in different projects. I don't mind paying something for it if I have to. I don't understand why they do it this way, put an advertising banner on it but then not answer their contacts.
Edit: I see now in the second link above, somebody just added this information:
Send a mail to email@example.com
asking them to remove the watermark stating the reason you are using for (project/thesis/commercial). They will ask you to mail the output of uv4l driver serial number provided you give credits in which you use it for.
Maybe I'll try to contact them again, but my project deadline is in a few days...