If you have nameplates enabled, it would be possible to communicate an enemy's GUID and affix an icon to the nameplate... but it would be quite limited in that (a) you would have to be in nameplate range of the enemy to see it, and (b) the icon would only be shown after you moused over the nameplate, and each time you went out of range, you'd have to mouse over the nameplate again to get the icon to show up.
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