The 990's remote is a UEI Catalyst48. UEI makes a lot of remotes, including the Radio Shack and One-For-All "JP1" remotes that are documented at Hi-Fi Remote . I'd check there for possible additional codes.

If you get really bold (and don't mind potentially voiding the warranty on the remote) you could even try the JP1 interface. I've got a co-worker who is experimenting with it at the moment, but I've never tried it myself.