Just how much work do I have to do accounting for different target platforms to make this a reality?

The problem here is not your different target platforms, really -- it's the fact that you're trying to use two different libraries. Are the interfaces to all the functions the same? As Scott suggested earlier, if they are the same, then you shouldn't have a problem. If, however, the public API has changed, then you're going to have problems. That's nothing to do with Java.

You mentioned an Ant script that handles it. Do you have a good link for Ant docs/tutorial? Of the first four I've looked at I seem to have a choice between well-written or complete.

[link|http://ant.apache.org/manual/index.html|http://ant.apache.org/manual/index.html]

The fact that something like Ant even exists suggests to me that WORA is more marketing than reality.

Er, no. Ant is a build tool. You write some XML, install Ant, and it builds your project for you when you type "ant build". It handles all the "javac -CLASSPATH=a;b;c;d org.drook.MyClass.java" and all the other commands. It's like Make, but sexier.