That's exactly what timer UIs are for. You just have to know the timing of the spawn to make it work. You can't input a timed delay on-the-fly. you have to have it hardwired in the GUI beforehand.
For instance, if you're using the timer mod from
http://www.horizonsoutpost.com/eq/timer.asp , when the mob dies, just move the slider to where the bar is currently at. Then you will know next time the bar reaches that slider point, the mob is about to spawn.