I love answering "A or B?" questions with "YES!"! That's happening a lot here at Schmartz, especially around lunch... Mike, what are you having, chicken or pizza? Mike says: "Yeah..."
With the next URC software update, we're going to be listed in their database. So that would be one way of programming your remote. I believe the update is due sometime this month.
Also, and I am not sure if MX-500 has this, you can use the PRONTO codes in hex format. If all else fails, you can use the PS2 remote, and program the Discrete Off macro on the remote, rather that use the one on the device. You could also use the PS2 remote for most of the codes, and just use the PRONTO hex code for Discrete Off.