If your OCR is unable to recognize the characters on a full page, I don't think it when scanning a region either? Unless using full resolution of a full image i

If your OCR is unable to recognize the characters on a full page, I don't think it when scanning a region either? Unless using full resolution of a full image is somehow too much for the algorithm in use. But then I'd just subdivide the entire image into regions, and scan them all independently. This is also a trivially parallelizable task, so you can throw many servers at it, if the time to get results is an issue.