Per-database login form using Web Site Rules and DOMCFG

We've set up a Web Site (in the Internet Sites view) for a certain URL to serve a variety of web applications.
Most web applications under that site need to use the same login form, which we've configured in DOMCFG.NSF to map the URL to a publicly readable login form.
For a couple of applications, we need to use a custom login form, so in other words, we would really need to configure per-database login forms rather than the per-virtual-server settings that DOMCFG allows.
I'm conscious that DOMCFG does not provide this, so I started hacking away with Javascript redirects.

My idea was that the standard login form could check the RedirectTo field to see if the user is going to one of those special web applications which require a custom login form, and redirect to that login form by doing a top.location.replace().

I can get the redirect to work to e.g. www.google.com, but if I redirect to the login database (DB ACL: Anonymous = Reader, Login form: $PublicAccess="1"), Domino still presents me with the original login page, but it has just changed the RedirectTo value to that second login form....!

It seems as if Domino is enforcing the standard login page for that virtual server, even though I am pointing it to a publicly accessible login page...!
Can I somehow circumvent this behavior, i.e. point to the login page that I want?

Or is there another way to show a login form based on the database the user is trying to access on that virtual server?

Thanks