The only abandoned cemetery I know of is the Mifflin United Presbyterian church cemetery on Rt 885 right behind the County Airport. The records are supposedly held by the Elizabeth Historical Society--412-754-2030. I believe burials here were from around 1802 to 1940's.

Your best source may be the Mifflin Township Historical Society. They have a listing of obits for the area. There is a Charles Gibbs-Nov 18, 1882 in the Homestead Times. The whole area was Mifflin Township then--I have found many of my West Mifflin ancestors in the Homestead papers. The great thing I have found about obits in these papers is about 90% of the time they give the cemetery.

The contact person for the Mifflin Hist Soc is Jim Hartman, he is very nice and helpful. You can get his email and phone from the website. If anyone knows about any old abandon cemeteries it would be him. Here is the link to their Obit page--you can check the whole list for your names. Scroll down the gray side bar for Jim's info.
http://mifflintownship.org/obituaries/

The Carnegie Library in Pgh has the Homestead papers on microfilm.