The Royals are now unbeaten in their last eight trips to Oakwell since 1996 but should have gone on to collect maximum points, with leading marksman Lucas Joao guilty of a runaway candidate for miss of the season after Mowatt's equaliser.
Earlier, Barnsley managed the first shot of the match with less than a minute on the clock as Mowatt volleyed wide from the edge of the box.
The visitors quickly settled, however, as Yakou Meite squandered an excellent opportunity to open the scoring after eight minutes.
Following a mistake by Tykes defender Mads Andersen close to the halfway line, Meite charged clear through the right channel only to lift a side-footed effort well over a relieved Brad Collins' crossbar.
But four minutes later, the Royals led when Andy Rinomhota's lofted long ball exposed a flat-footed home defence.
Ejaria expertly latched on to the ball without breaking stride and found Collins' bottom-left corner from 15 yards.
In response, Callum Brittain's right-wing cross glanced against the visitors' crossbar and the same player's thumping volley was bravely headed away from goal by Moore, before the final action of the half saw Romal Palmer head over from Mowatt's subsequent corner.
But, after Palmer dragged a 20-yard opportunity wide, it was Reading that came within inches of doubling their lead.
A rapid counter attack saw Olise and Joao combine to tee up a charging Meite, who just failed to get a stretching leg to the ball in front of Collins' goal.
Mowatt aimed the resulting spot kick down the middle of the goal and was a little fortunate to score after the ball struck Cabral's trailing leg.
More fortune was to follow for the hosts when Joao's horrific miss staggered everybody in Oakwell after a mix-up in the home defence led to Collins making a last-gasp challenge on Meite.
The ball then ran loose to the Royals' top-scorer but he inexplicably side-footed wide of an empty goal from 12 yards.
Barnsley immediately looked to punish such profligacy with Styles shooting over from 15 yards and Andersen stabbing against an upright after Cabral dropped a high ball into the box.
Morris also cut in from the left and forced a save, while Aapo Halme headed over from a corner and Dike dragged wide from 15 yards, but Reading held on for a share of the spoils.