Tis the winner , eucalyptus oil won't damage the finish of the keg.
Dab or wipe some on leave it for 10 mins out of the sun and it'll soften the label and glue nicely,reaply if needed but elbow grease shouldn't be needed . Reapply again if needed.
I use it at work to remove sticky labels etc from polycarbonate sheets with no dramas.