Your "We are no longer accept registration for this event" idea is a simple, elegant solution. If you haven't figured out how to edit this yet, go to the language files (in the back end, click on Event Booking under components, then click on "Translations"), and scroll down to find the "We are no longer accept registration for this event" text - which is a poor English translation and needs to be fixed anyway on English websites.
Just add the text you'd like in the box and you're done. I believe you can insert simple HTML tags, so making this a link to a wait list form should be fairly simple.
The fact is, there are so many variations and needs for event bookings (from classes, to concerts, lectures etc, that creating a "wait list" that will satisfy all of his customers is a near impossibility. You can waste $100+ on DT Register, only to find that their wait list doesn't work the way you want it to - of work with the much more simple and elegant component we have here, and make some simple customizations yourself.
FYI, I'm sure the developer would customize this for a small fee, and maybe create a wai list that works exactly as needed for your events...
