Another Idea to accompish the same point (using panels)

Issue Summary

Instead of using a module ... just do this.

Set your 403 page to use the url "foo".

Create a page panel using the panels module.

Make it use the url of your frontpage, in this case "foo".

Make it a 2 column layout.

On the left column put your "You must register/login to use this yadda yadda"
On the right column insert the login block.

Just a suggestion ... It's what I use on my sites.