I see no reason why not.
In terms of logic, it works. Death Attack is all about examining your unaware (or unmoving, in this case) opponent, and seeing exactly where you could pinpoint a precise attack. With Time Stop, you can easily examine your opponent's weak spots, aim precisely and perform a Death Attack.
In terms of balance, again, no problem. The wizard with Time Stop is going to launching off barrages of Wail of the Banshees, Meteor Swarms and the like, so there is no reason why the assassin cannot use UMD to have a Time Stop to bolster his powers.