Dear Christian,
first of all many thanks for your prompt and helpful response !
Check:
Hope this helps.
It didn't help directly, because on the site I'm building, users won't be able to register directly. Indeed, registration will be possible only once users have provided some docments. So what users will do is a registration request. But your answer was still helpful in providing excellent starting points which led me to http://drupal.org/project/webform_validation which is, I think, exactly what I need. I didn't find out how to validate against a checked checkbox yet, but this module is obviously the good framework n which the needed validation rule shold be added, if not already there.
Thanks so much for being so helpful ! Best wishes, Sherab.