The promoted Reds will arrive at the John Smith’s Stadium on Saturday seeking a third straight win after also beating Derby County 2-0 at home.
Josh Scowen put a last-gasp winner past QPR’s former Town keeper Alex Smithies on Wednesday.
Barnsley took an early lead through Marley Watkins’ free header at the back post.
QPR turned the game around with two penalties, as Tjaronn Chery fired down the middle two minutes into the second half and Sebastian Polter placed his strike into the corner.
Conor Hourihane’s spectacular free-kick levelled on 77 minutes and Scowen finished on the half-volley before QPR had Grant Hall sent off.
Wagner will be pitting his wits against Paul Heckingbottom, who guided his hometown club to League I play-off success last season.
The 39-year-old former Reds player replaced Lee Johnson at the helm in February.
As caretaker boss, he guided Barnsley to Football League Trophy glory, with Oxford United beaten 3-2.
The Reds returned to Wembley to beat Millwall 3-1 in the play-off final.
Heckingbottom was given the head coach job on a permanent basis in June and has boosted a squad in which former Town wideman Adam Hammill is a key figure.
Barnsley splashed out £600,000 on Walsall striker Tom Bradshaw and £500,000 on Colchester United midfielder George Moncur.
Midfielder Ryan Kent has arrived on loan from Liverpool and striker Stefan Payne from Dover Athletic, where he was a prolific scorer.
Heckingottom has also brought in keeper Nick Townsend from Birmingham City, defenders Angus MacDonald (Torquay United) and Andy Yiadom (Barnet), French midfielder Sessi D’Almeida (Paris SG) and striker Elliot Lee (West Ham United).