How should an Ant <fileset> be created programmatically from another (custom) Ant task?

Hello,

I've developed a few custom Ant tasks over time, but I need a little help
doing something specific to Ant's data types.  I've created a task that
retrieves a set of files from a server, but the names aren't known in
advance, they're determine during execution of the task.  I'd like to
create a fileset that has an ID and can be referred to later on by other
tasks and targets in the build.

This is how I assumed I'd get started:

FileSet fs = new FileSet();
fs.setId("someid");
fs.setProject(_project_);
fs.setDir(_dir_);
fs.createInclude().setName("entry/within/dir1/containing/file.abc");
fs.createInclude().setName("entry/within/dir2/containing/file.xyz");

I've some doubts about the "createInclude" logic... but I haven't got that
far yet.  I'm stuck on the "setId()" part which just doesn't exist.

In a build script, I'd write:

<fileset id="someid" dir="somedir">
  <include name="relative/path/to/file" />
</fileset>

... then refer to it later with "refid".

How can I get this to work?

Thanks,
Christopher