I had an internship with the BPD for a semester. I was able to get it -1) becuase I "knew someone" and 2) becuase I was a college student and the class required an internship with a police department.
I met a lot of great BPD guys and they told me that it wasn't unheard of for things like ride alongs and internships; BUT you really need to know someone. One of the officers I was working with told me that he had tried to get his girl friend a ride along(not with him obviously, but somewhere in the dept) becuase she was interested in becoming a police officer herself- long story short, brass wouldn't let her go for a ride along after repeated attempts....so he couldn't make it happen and he worked for the department.
But like they told me it's not unheard of, so it wouldn't hurt to start calling out favors or calling depts and asking though. good luck.
It depends on the Captain for the district, each one is very different. If you are serious about it I would go to a Community meeting for the district or neighborhood you live in and ask the Community Service Officer or Sgt about it, they are the ones more likely to ask the Captain versus calling the busy front desk officer who will broom you somewhere else. Its not unheard of but you do need to know someone so go sell yourself. My roomate in college went on a ride along in Dorchester because his professor was a former Captain and he took a liking to him.