I've been hunting google for hours and I'm satisfied I know how to build a custom user registration module.  What I can't find out how to do, is to tell DNN to use my custom register page, not the built in one.  I can see how to do that for the login page, but cannot find how to do it for register.  Can someone please help as I'm starting to cross eyed reading so many articles that aren't helpful.

If you want a custom registration page, create a page and place your module in it, then login as admin and from the site settings, under the "Advanced Settings" - "Page Management" there is a setting for "User Page", change it to your custom registration page.

Hope this is what you are looking for.

yup - that's the setting.  Saw that there but the wording of the title didn't give me the confidence that it was the correct entry.  Perhaps the DotNetNuke guys should either improve the wording of the title from "User Page" to "Register Page", or enhance the help text to reflect that this is the page that provides user registration.

Anyway I created a placeholder page and linked the user page to it.  When I click on  the Register link on the login page (our site doesn't use the register link from the page heading to avoid confusion with a recruitment register page we have which has a completely different purpose), I get an error message

Description: HTTP 404. The resource you are looking for (or one of its dependencies) could have been removed, had its name changed, or is temporarily unavailable.  Please review the following URL and make sure that it is spelled correctly.

Requested URL: /proma_PortalRegister/tabid/160/returnurl/ /Default.aspx

As you can see, the URL has been incorrectly formed.  Maybe I'll just stick to linking to the custom register page from a custom login page where I can control everything.... :(