PowerPoint doesn't give access to its status bar. A small modeless dialog box used as a progress indicator is another possibility. Or if your add-in has ribbon UI, some kind of change to the ribbon might suffice.
And on the bright side, most PPT users won't be expecting status updates on the status bar.
Even MS makes very little use of it, so people tend to pay little or no attention to it. They probably won't mind (or even notice) if you don't use it either.