Can a Drupal webform be used to create a waitlist registration?
I do think that this is possible it also depends on the event configuration in CiviCRM. 

If it is not possible you could create a webform in which each participant gets the status on waitlist instead of registered. That way you could create a waitlist.

To enable Participant Status - on waitlist in your webform -> you need to visit: /civicrm/admin/participant_status?reset=1 and enable both On waitlist as well as Pending from waitlist