Ok, I don't like to listen to "can't do it" answers because such answers are wrong pretty much 100% of the time. Anything can be done. The question is, the extent to do it.
However, you do provide a good alternative. If I do make a PHP page I should be able to use javascript to build a URL into the submit button based on options on my custom PHP page. Which, when submitted, should be able to build all the user selections into a full URL. I wonder however if it's possible to do such a thing with the following sections:
* Username/password for new account
* Registration details
Also, it might theoretically be possible to build a "mock" summary section at the bottom based on what the user selected ("mock" as in, it does not pull the real database queries but just pulls data from the page to build a mock summary).
Well, I will work with this, and see what I can come up with. A bit late tonight though to begin on this.
UPDATE:
Looks like you can use the API to create a new user, password, and registration, via PHP. http://docs.whmcs.com/API:Add_Client
It's looking like it might be possible to, using the API, combine the complete selection and registration all into one page.
Time will tell, but it's looking optimistic.