Lee, I'm not exactly sure what you are after. Do you mean internal modifications or external modifications, the latter being in the area of isolation (anti-vibration), power cords, and the like? The Hi-Rez Highway board is a good place for discussing this sort of topic (internal and external modifications). If you are after internal modifications, I know of three people who have performed all sorts of modifications on Sony SACD players, including the '777ES. They are Ric Schultz, Dan Wright (www.modwright.com), and Richard Kern. Ric and Richard post on the Hi-Rez Highway board from time to time. Ric uses his real name, and Richard posts under the moniker, "Audiocrazed". I have no experience with the services Ric, Dan, or Richard provide, but many people on Audio Asylum have used them, and all three get high marks. Go over to Audio Asylum and search the site for a lot of information on internal modifications, and then ask some questions there. There are a lot of people here who can help you.
EDIT: I can't say that Ric, Dan, and Richard all modify the '777ES currently, but all three modify SACD players.