There currently is no requirement that the web address go on the title page. If you feel it is necessary it could go on the line under the address of the publisher. There is also no provision for putting it anywhere else. The Formats chair says that the rules require just a few things from that print page: the ISBN, copyright date, and publisher name and address.
The line under the web address is not an underscore. It signifies that that section of print is a link to another page on the internet. Once you turn that address into braille it is no longer a link. I think you could cover this on the transcriber's notes page (In print, web addresses are underlined to indicate a link - or something like that). I have contacted the BANA Computer Braille chair to see if they have developed a standard way to handle this, and I will return to this question when I get an answer. If you look at this page (forum) you will see underlined text indicating a link.