I think I may have figured this out - gonna give it a shot later today (assuming of course my Tv is actually delivered :))
If you have a Radio Shack or One For All remote I believe you can use advanced codes. See this site (DLP codes listed at the bottom):
http://www.hifi-remote.com/cgi-bin2/ueic.cgi?TV_0060
Here is what I'm planning to try with my OFA 6131:
1) Program the "TV" function of the 6131 remote using 0060 (the Samsung device code) and then teach it the various advanced codes from the site above (discrete on/off and each of the inputs):
2) Once these discrete codes are on the 6131, I can then teach them to my MX-500.
Should work, right? I guess the big question is, does the 6131 accept advanced codes and if so, can I use any of the buttons in the TV mode?
Hopefully I can give it a shot later and report back. Much cheaper solution (esp. since I already have the remote) than buying that IRClone setup to get your MX-500 to accept discrete codes (even then I'm not sure if it will work).