There probably wouldn't be enough Internet addresses to go around if they were. Geez, I learned not to let that happen decades ago writing embedded assembler code for typewriters. Exposing all these HTTP servers on the Internet is the hard part. I contend that addition of a subsetted HTTP server on the clients to catch INDP notifications would be included in the client application that handles the notication and would be a relatively small amount of code considering everything else the client application has to do. There is a great difference in cost, complexity, resource consumption, and security considerations between running a client on the Internet and deploying a server on the Internet.
Most Internet servers are used by many users, so the cost is affordable.
A server per user just won't scale to the Internet. Peter Z. has a suggestion for helping to determine what is supported. However, this ignores the fact that INDP also handles human readable. Following the SF meeting I would like to formally propose the following.