Ended up writing a category which solves this problem using the customization capabilities of libpng. Posted a gist here: https://gist.github.com/joshcodes/5681512
Hopefully this helps someone else who needs to know how this is done. The essential part is creating a method
void user_read_data(png_structp png_ptr, png_bytep data, png_size_t length)
{
void *nsDataPtr = png_get_io_ptr(png_ptr);
ReadStream *readStream = (ReadStream*)nsDataPtr;
memcpy(data, readStream->source + readStream->index, length);
readStream->index += length;
}
and using
// init png reading
png_set_read_fn(png_ptr, &readStream, user_read_data);
as a custom read method.