I don't know much about Java and I haven't done anything with Java and Hiawatha. But from what I've just read about AJP, I think the easiest way to use AJP with Hiawatha is to make a FastCGI application which can connect to a servlet container via AJP.
Implementing FastCGI was a complex process. To add AJP to Hiawatha will make things even more complex, so I don't recommend it. And by making a FastCGI-AJP wrapper, you have a webserver independent sollution. Which I believe is better dan creating an AJP-patch for Hiawatha.
Writing a FastCGI application is really easy. Here is an example C program
. Just add your AJP code to it and use FastCGI in Hiawatha to use it.