Hi Peter,
I'm no expert on DNN and I also had to learn some things the hard way.
When we decided few months ago to use the DNN platform for a public website we mainly considered and benefits of security and administration. If I had to develop my own website with vanilla ASP.NET my life in many cases would have been much easier :)
So currently I take what I can from DNN and in the other cases I try to work around it.
For example, I was trying to embed a video player (flash based) in a module but I wasn't able to make it work. My solution was to put an IFrame inside and call a regular ASP.NET page which works fine, but with a lot of tweaking.
So the registration issue seems like a problem for many developers who wish to customize this page.
For me, I saw no other way but to go into the source code and modify it.
I will have to do it again if we decide to upgrade DNN, but that's something that I'm willing to live with right now.
Another thing which worries me is performance. It seems like DNN is putting a big overhead on page rendering, but that's something I will need to test in a lab once the code is complete and then do some improvements.
Also, and this is a big "unfortunately, I am dissappointed with the level of support. The DNN 4 book is no help in many cases (too generic), many questions in the forum are left unanswered. Maybe this will improve in the future when the DNN community grows, I really hope so.
When you face your CTO you have to put all the facts about this platform, including the benefits, and they don't come for free.
Good luck with the project!
Ofer