As IATA members they should have an interlining agreement, though getting them to honour it can be a different matter. Generally I have found IB pretty good about this kind of thing, but staff at some of the outlying airports can be less easy to deal with.

IB will not be able to give you boarding cards or check you in for PHL-PIT.

This should not matter for getting the baggage tagged though. All the IB staff need to see is a confirmed reservation on the onward flight. Preferably get US to print you an official looking itinerary/eticket receipt before you leave for OVD. This is likely to get you further than a home computer printout.

Make it extremely clear to the IB staff that you are not expecting to be checked in for the flight. Instead, you just want them to add the PHL-PIT flight to the baggage tag. This can be done manually through the computer system. It is not a security issue because you have to clear customs at PHL anyway with your bag, and US will not accept it for carriage unless you have been checked in by them.