When I have been wanting loot off of a rare on the Timeless Isle, I have setup several "Pizza Timers" macros using DBM.
You can go to wowhead.com and find many of the approximate spawn times for each rare elite. So, for example Hulon has roughly a 40-60 minute respawn time. You can make a macro like this:
/dbm timer <time> <name>
What this will do is create a countdown timer for 40 minutes with the name Huolon. The /in command must be followed with the same amount of time. You then specify what channel you want to have the spawn be announced (helps so you can see it more clearly). I picked /officer, but you can use anything. Even send a whisper to yourself (/whisper your_name Houlon About to Spawn!).
/dbm timer 40:00 Huolon
/in 2400 /officer Houlon About to Spawn!
You can setup multiple timers, so you can track many different spawns.
Also, you will need to know when the last time it was killed to "accurately" determine when the next spawn should occur.
I hope this helps =)