"Of course there are thousands of documents about the file format itself, but nothing about the data bytes."

That would be because Intel HEX is a generic format, i.e. it is not restricted to the 8051 family. What you want to do is called disassembly. A search on that word will give you several references to programs that actually do this for you. You will also want to read the 3 80C51_FAM documents found at: http://www.semiconductors.philips.com/products/...other.html
Among other valuable information they list the complete instruction set.

Best regards,
Rob.