Look you need to get with the program. Data can be sent by GET or POST or both. You may or may not need to send a session cookie or other session data. If it is HTTPS it is encrypted the moment it leaves your browser so you can't capture packets, if not you can. Some browsers let you look at what the browser is doing in more detail than say IE. Get a copy of Mizilla Firebird and have a look at Tools|Page Info which will show you all the forms and form fields and Tools|Options|Privacy|Cookies|View Cookies to see cookies including memory resident ones.

It's a pain in the ass to go through each registrar's registration page, and do a view source.

You are breaking my heart. That is likely what you will have to do most/all of the time. Every time they change their API you code will typically break as well BTW. Some registrars offer back end APIs.