QA-EQU-2: Are remaindered discs (X-ed out discs, factory seconds, hot stamp rejects, etc) of PDGA-approved models legal for use in PDGA competitions?

Yes. They are legal for PDGA play as long as they also meet the overall restrictions (weight, rim sharpness, flexibility, etc) as outlined by the PDGA Technical Standards document. Players always have the right to question the legality of a disc used in competition. In such cases the TD will make the final call.