It would be possible to draw a reversed progress bar on newer releases of
macOS (for which the HIToolbox is not used).  It would be a bit of work because
the colored portion has two rounded corners and two square corners.  Probably
it would be easiest to apply an affine transform that reflects across a vertical
line before drawing.  That might even work with HIToolbox if the graphics
context were modified before calling the HIToolbox routine.

However, I do not think such a thing belongs in the Aqua theme.  There is no
reversed progress bar available in XCode and no mention of such a thing in the
Human Interface Guidelines.  So no "native" macOS app would ever have a reversed
progress bar.

To be honest, I don't recall ever seeing a reversed progress bar on any
platform. That doesn't mean that they don't exist and it doesn't mean that Tk
shouldn't have one.  But I am quite certain that the Aqua theme should not have
one.