The £2.6million summer signing netted just his third goal of the campaign with only seconds left to cancel out David Mooney's second-half header.
An upset looked on the cards when Mooney nodded in his ninth of the season with 12 minutes to play and the Os were worthy of their lead. But it was snatched away from them three minutes into stoppage time when German striker Proschwitz headed in Tom Cairney's cross.
The visitors went close after 16 minutes when Kevin Lisbie went past two defenders in the box before shooting wide. And they were denied by Eldin Jakupovic's excellent save from Leon McSweeney's long-range drive soon afterwards.
Hull boss Steve Bruce switched to a 4-4-2 formation in a bid to offer a more potent attacking threat in the second half.
It almost paid dividends within three minutes of the restart, but Alex Bruce failed to keep his header down after connecting well with Cairney's delivery.
But it was Orient who should have taken the lead with 54 minutes gone. Lee Cook raced through and had the goal at his mercy but could only shoot straight at Jakupovic.
They were almost made to pay two minutes later as Paul McShane rose highest to meet Cairney's corner but cleared the bar with his header.
Young midfielder Dougie Wilson was handed a first Tigers appearances with 15 minutes remaining, but it quickly turned sour. Dean Cox delivered from the right for Mooney to head home from six yards.
Lisbie headed wide in Orient's next attack and Proschwitz volleyed off target late on as it looked like City's final chance had gone. But Cairney had one more pinpoint cross to deliver and Proschwitz was on hand to provide the decisive touch.