Is loan forgiveness by working in underserved urban or rural areas much of an option once you complete pathology residency? No it is not, in fact it is specifically disallowed in the state of California.
Do some private practices in nice areas ever offer to pay off loans in addition to providing you a salary? No, there is no incentive whatsoever to do this. The job market is such that just getting a paying job is challenge enough for most graduates not from a top 3 program.
In case private hospitals or groups in non-underserved areas do offer to pay off your loans, do they necessarily offer you a lower salary and decreased benefits package? Again, no. While some very rare groups do pay off a small part of one's loans, this is almost always funded through the hospital, is partial and often in an extremely bad place to live and is still the exception. Meanwhile someone could live in Malibu and have a family practice in Inglewood and get loan repayment in California.