I can envision two scenarios-
Maybe as a cool alternate way to connect a 9V battery box which would eliminate any additional connections in the signal path between mics and recorder. The battery box wouldn't need anything like a transmitter or second recorder attached down stream of it.
OR
Potentially passing through the higher PIP voltage through from a secondary recorder?.. but I'm am unaware of any recorders that produce more than 5V PIP natively, so that higher voltage would need to be supplied from an XLR adapter / PFA in the second recorder.
However, even with a non-US version of the PR-2 those scenarios are not assured to work, as the pass-thru path might be capacitor coupled, just like it is through a battery box, in which case it will pass signal but not DC voltage. Deity's answer didn't indicate if it's actually possible with a non-US version or not, only that it isn't with the US version.
Blakenan, even though you can't plug the battery box into the PR-2's headphone output and have its voltage passed through to the mics, you can still use your 9V battery box in the typical way between the mics and the PR-2.