The records in Przemysl and in Sanok are held by state archives (archiwum państwowe) and not civil registry offices (urząd stanu cywilnego) - maybe that is why your relative did not received any information from them. Then in state archives in Przemysl there are two fonds containing records from Plonna parish one is "records of a Greek-Cathoic parish in Plonne" (Akta stanu cywilnego Parafii Greckokatolickiej w Plonnej), fond 2082 and "archive of Greek-Catholic bishopric in Przemysl" (Archiwum Greckokatolickiego Biskupstwa w Przemyślu), fond 142. The second contains the records for Wysoczany. Mybe, if your reltive asked in the right archive he/she asked just to search the records for Plonne parish? Or maybe he/she asked the right question but they did not search the right set? You may consider asking them once more with the above information? It is of course most propable that they searched both sets as it is quite good archive. Are you sure that they were from Wysoczany? What is the source of information? Or maybe they were of different religion at that time?
I do not think you would find anything in Ukrainian archives on Wysoczany, although I am not sure what they have.