I will give the disclaimer that I am only an Undergraduate so interpret what I say as you will. I have somewhere around 2500+ hours now of clinical experience with my local plastic surgery team and some of the senior residents have previously asked me for my opinion on elective students.
I will say that the big thing, regardless of the specialty, is to be knowledgeable in it. Universities or large academic centers sometimes have webpages giving a brief bio of each staff surgeon stating what their focus is and this Information can be useful if you know who you're working with on your rotation.
To further refine that first point, I'd say that common things that I've seen is not taking a focused history, not knowing the dominant hand and occupation in hand injury pt, not looking inside the mouth in a patient with suspected facial fractures, not using both hands to operate (ie using only one hand to cut sutures [Typically with a metzenbaum or mayo scissors] when both are free) and not being able to interpret facial CTs for fractures (ie Le Fort 1, 2 or 3, ZMC fracture, orbital floor fracture, etc) or not assessing cap refill or nerve function in a finger lac and a number of other things that I cannot recall at this time.
For most nuanced things, your best bet is either asking the residents at morning rounds or buying coffee for the clinic nurse or other support staff as they will probably know what that certain surgeon wants. It's quite rare for someone to go into an elective knowing something like what prep a certain staff surgeon likes so it's best to always ask the residents or nurses in the clinic what they like done.
Beyond that, the number of Plastic Surgery residents that I've encountered, only a few had a graduate degree going in, some decided to peruse it once in residency but, at this time, It's not required.
I will end this by saying that of all the Mac elective students that I've encountered, a number of them have matched into plastics and, personally, I'd attribute it to the fact that you can see that they have the knowledge and are willing to learn more, in addition, they are truly passionate about plastics. You are still a few years from being an elective student but doing plastics research can't hurt, as Shady suggested, if you can reach out to Plastic Surgery residents at Mac then that would be the best option as I know a number of them went to Mac. If you wish, you're free to DM me as I regularly interact with a number of plastic surgery residents and staff.
Best of luck in your future endeavors,