I don’t think this is a chrome level feature that you can play around with. As the “file picker” is system based, rather than something directly managed by chrome. So if you went onto a different operating system, used the same website code (say where you can upload a file) with the same chrome browser version, the dialog that opens is totally different.
I’d look into how to customize the kubuntu file picker/file manager dialog itself. I believe the file manager is called nautilus if I remember correctly. I’m not sure if you can set the default directory, I think its always just the home directory, but you might be able to set folders as favorites which are easily selectable.