A little late to the party, but I'm with Martin: assuming that your bounding boxes are defined as quadralaterals, you should be able to translate the OCR output into the existing facsimile/surface encoding, though I've not heard of any automated way of doing so. Couple that with a resp attribute pointing to a responsibility definition of your OCR engine and a certainty attribute reflecting the recognition confidence and you're in business. If the OCR output is defined in polygons... no such luck.
Encoding the output of multiple OCR engines, meanwhile, is an entirely different animal. Even with the resp and cert attributes, I don't think TEI really lends itself to the encoding of disconcensus.