Can i set a widget (like a button, for example) hidden in Qt Designer?

[Lykurg]

not exactly, you can add dynamic properties in the Qt Designer: in the Property Editor there is a big green "plus" button, using it you can add different properties

I've tried that, but when I was to add a Boolean "visible" it complains that that key already exists. (Although it is not in the list.)

[spirit]

weird, looks like a bug.