It does lag a bit, meaning, the label won't update instantly. You might have to play with the updateinterval options for xlabel, or xtaskbar... or was it updatealways in xlabel... I forget. You'll have to experiment.

(and about the xtaskbaronenter method - I think i remember trying that when I was working this out a while ago, and I also found that wouldn't work. I'm assuming it's because the onenter is fired before the evar is available/read.)