Why can't I perform your first suggested method of doing this?
Because the ant plugin, AFAIK, is a collection of ant jars. When I do
the File>Import...>Plug-ins and Fragments and import org.apache.ant I
get a plugin project with a lot of jar files in it.
You still have to replace the Ftp.class file in the correct jar.
There's no FTP.java file to edit in the eclipse plugin, although the
plugin does contain source zip files.