It should be possible, in any language of Wikisource, to check all existing text against a known dictionary valid for that year, and to find words that are outside the dictionary. These words could be proofread in some tool similar to a CAPTCHA. They might be uncommon place names that are correctly OCRed but not in the dictionary, or they could be OCR errors, or both.
Has anybody tried this?
Such finds are not necessarily the only OCR errors. Some OCR errors result in correctly spelled words, that are found in the dictionary, e.g. burn -> bum. So full manual proofreading and validation will still be needed. But a statistics based approach could fill gaps and quickly improve full text searchability.