I don't think there is a web.config setting for this, but we have dealt with this in a couple of different ways:
1. Capture the request in global.asax for that page and redirect to your page
2. Edit the out of the box page
3. copy the out of the box page to a seperate project, make your changes and place an post build event that does an xcopy and copies your page to the website project.
- this works well if your client will be doing a rebuild of the solution prior to deploying the sitefinity upgrade, this is alswasy a good idea as API calls can changes sometimes...