AJP1.3 is a straight request forwarding protocol. There is no concept of caching contained in the spec. Since primarily, it is designed to forward requests on to a dynamic content server, it is somewhat difficult to have a fast, lightweight connection protocol that also remembers and understands the difference between static content returns and dynamic content returns.
Most likely you would need to either remove the load from servlet container and deploy it on the front-end Apache, and/or configure a SQUID proxy to perform caching in conjunction with mod_expires. This becomes somewhat complex, and I think expends as much time as you are trying to conserve.
The best option would be to serve static content from the web-server, assuming that you have two servers - one for JBoss/Tomcat/Jetty and one for Apache. At least, you will be sharing the load.
Someone may be able to suggest a more effective scheme.