/echo is your friend

Wont display a time on the icon as you desire but you can have a message display to you in chat log that said ability is ready. Sadly /echo is all one color so the disadvantage is that if you have multiple /echos your chat log might annoy you. However you could offset it buy having the message displayed written in your on form of "short hand" so to speak.

As an example I use:

/ac flaming arrow
/wait 60
/echo !! -- FLAMING ARROW READY -- !!