I was looking for a UI framework that could easily embedded into other applications. I'm doing all the communication with Wayland myself, then create an OpenGL context from the Wayland surface which Slint renders into. Also Slint has this language you can create UI's with, which I think is not too difficult for users to learn so anyone can customize their lock screen easily with it.

Yeah sure. Well slint is a whole ui library and so you can customize basically anything. You could add extra text, move the clock and the password prompt wherever you want. You could even remove the password prompt and replace it by something else. Here's the config of the screenshot above:

import { LineEdit , TextEdit} from "std-widgets.slint";
export component HelloWorld {
    in property clock_text;
    in property checking_password;
    in-out property password <=> password.text;
    callback submit <=> password.accepted;
    forward-focus: password;
    states [
        checking when checking-password : {
            password.enabled: false;
        }
    ]

    Image {
        width: parent.width;
        height: parent.height;
        source: @image-url("wallpaper.png");
        HorizontalLayout {
            VerticalLayout {
                alignment: end;
                spacing: 10px;
                padding: 40px;
                width: 350px;
                Text {
                    text: clock_text;
                    horizontal-alignment: center;
                    font-size: 60pt;
                    color: white;
                }
                password := LineEdit {
                    enabled: true;
                    horizontal-alignment: left;
                    input-type: InputType.password;
                    placeholder-text: "password...";
                }
            }
        }
    }
}