No point. It is too late by that time. Most programs know who they want for most/all of their spots even before interviews, and they decide ranks and any undecided spots and main backup ranks right after interviews (usually immediately after they wrap up, while the key persons are still together).
You should have sent a handwritten and specific thank you note/card if you enjoyed the externship. That will put your name out at the program one more time and will express gratitude for the learning month. After residency interview, you are too late and clearly pleading for the program (and any program that would be susceptible to that or change their ranks at that point will likely be available in scramble anyways).
I do that stuff sometimes (thank you note or pkg for visit or interview or etc), but I only do it when I legitimately appreciate the experience or assistance or whatever. I sent my new office credentialer a present for doing all of my hospital and insurance apps successfully and efficiently last month. However, if you are just doing it to every place to try to gain something in return, they tend to think you are trying to compensate for lack of qualification and will usually see through that (although it may still work a bit... due to human norm of reciprocity).