Given the above, I see two possibilities.
The first involves having coreboot build the standard tables.
Like I said why not just have a mini program between the two that handles this operation. It could be added to coreboot or legacybios as an option module. The mini program would read the coreboot tables and what ever else it needed and then set these tables up in a format that coresponds with legacybios, and finally passes the tables to legacybios. Does it have to be more complicated than that?