"My password manager fills out the registration form, of course, but it won't submit because it wants an email address, not a username."
Big UI pain: sites that are not compatible with browser-stored login info.
Big UI pain: sites that are not compatible with browser-stored login info.
Compare the login forms for Verizon Wireless:
Home page - https://www.verizonwireless.com (upper right corner)
Logged out page - https://login.vzw.com/cdsso/public/c/logout
Tweaking your password manager settings to log you in through a site's logged out page can often bypass the compatibility headaches.
Using 1Password's Chrome extension, this trick has worked for me so far, although your mileage may vary.