Well... It turns out that Martin's suggestion of using ImageMagick is probably the best option in my case.
There's a CI filter that does noise removal, but it's not available in iOS, and I will have to use ImageMagick to convert a PDF to TIFF for OCR anyway, so ImageMagick it is.
An alternative is the small image processing library that Chris Greening made. If you don't need the full force of ImageMagick it will do most of the light lifting for you, and some of the heavy lifting too.