Will a publication in one subfield of my discipline help me get into graduate school in a different subfield?
Even if your publication were in physics or economics or any other field, it would still be a positive component of your application. When I look at graduate applications, a publication record--while not essential--is valuable because it signals the following: 


Interest in doing research
Initiative to get into a lab and work on a project
Familiarity with what primary research is actually like
A close enough relationship to a faculty mentor for me to pay particular attention to that mentor's letter of recommendation


All of these hold in your case.