Ohhhh --- Yeah, it's definitely possible to do that.

There's only four progressbars that are used anywhere, though. The first is in the splash screen, the second is in the status bar while LimeWire is visible but still loading the core, the third is all uploads/downloads (they use a single JProgressBar) and the fourth is for the progress of search results (they also all use a single JProgressBar).

You can find the first two in com/limegroup/gnutella/gui/StatusComponent. The one in tables is in com/limegroup/gnutella/gui/tables/ProgressBarRenderer. The one for the search results is in the 'macosx' directory of gui, further in com/limegroup/gnutella/gui/AquaTab. (That one is easiest to change in com/limegroup/gnutella/gui/ProgTabUIFactory, though).