It would have to be someone who knew Star Trek pretty well, and had enough self-control not to kill, maim or otherwise interfere with Sue before their observation time was up. There must be enough extras hanging around for them to blend in for a good long while, as long as they avoided getting into a situation where Sue adopted them as her best friend or some other acquaintance (e.g. boyfriend from the Academy who still pines for her and will at some point press his advances so she has to be saved by the man of her choice). While there is precedent for that (in TOS with Andromielle) and agents have their own immunity to the Sue-spell, it would also draw attention to the agent and he or she wouldn't have immunity to any attempts made by the canon characters to get rid of him or her as an adjunct to Sue.
Not sure who would be most suitable, though. Is there anyone in Intelligence who might be able (and willing) to do it as a one-shot mission? :)