Maybe nickname counter could be solution???
Edit:
IF you want to use personalized away message just use "NICK ¤{Will be back in (!XCOUNT)}"
using my nickname counter plugin...
... (replace ¤ with the the ascii character 160.. do this by holding down the ALT button and pressing (with the numpad keys on the right of your keyboard) 0160 then release the ATL key again, and a space should appear, it's a speciel character enabling you to use personlized status...
..)
Hope this helps you...