Found this old thread, and have implemented it with some improvements.
Firstly, I also had the problem of the blank page with only the FMP banner at the top. This was solved by adding a closing tag to the end of the replacement text in iwp_auth.html.
Secondly, I've eliminated the need for registered users to enter via the public/guest page, and given then a completely separate URL to enter by.
I thought "surely it's not going to be this easy", but it was...
Instead of editing the iwp_auth.html file, create a copy of it called, eg, iwp_auth_guest.html, and make the changes from the OP to that new file instead.
Then in the URL, simply change the file name, and ta-da... the old URL still works for the login page, and the new URL works for automatic guest login.
This eliminates 2 1/2 of the 3 Cons in the original post (1/2 a con remaining because although you don't have to edit iwp_auth.html, the solution still has basically the same consequences, and therefore may not be suitable for some applications).
I guess it adds a new con though: may not be forward compatible - in future FileMaker may decide to make sure any HTML files that it doesn't know about do not work. Works fine for me on FMP 8 (not server).
(Not required now, but before coming up with this variation to the OP's method, I also made his password entry field use white text, so the password could not be visually snooped over the shoulder).
Wow... I've been searching for a way to use two URLs to log into the same database for Guest/non-guest logins for ages. This thread nearly solved the problem, and with a minor variation... it is now solved completely! Hooray!
This modification to the OP's original also makes the whole method so much easier to implement. There are now only two small steps required:
Duplicate the iwp_auth.html file naming the new file something like "iwp_auth_guest.html".
Change the content of the new iwp_auth_guest.html file replacing everything from "" (inclusive) with:
To use the database via IWP, normal registered account holders log in using a link to the usual login page, and guest use a link to the new login page (IE, in the URL substitute iwp_auth.html with iwp_auth_guest.html).
So simple!
Much thanks to the OP for getting me headed in the right direction.