The js guru suggested a kind of timer function that would check when the iframe had finished loading before running a script to get the values. But it turned out that the simplest solution was to make the .jsp page populate the values itself - you can make the content of the iframe control what gets written to the screen in the parent as much as you can do it the other (and more logical) way around. In this case the connotation of parent and child is wrong - they're more like siamese twins joined together and able to influence each others movements.
Another useful thing to know is that you can only make two apps on two different servers talk to each other if you suffix the server name in the URL with the name of the domain that they both share. So if my domain is www.markp3rry.com then you will need to set the servers to http://server1.markp3rry.com:8080/ and http://server2.markp3rry.com:8090/ (for example).