A list of undergrad colleges that the 2014-15 entering Harvard Law Class attended is at http://hls.harvard.edu/dept/jdadmissions/apply-to-harvard-law-school/undergraduate-colleges/ . Note that the 572 students attended 172 undergrad colleges, so everyone is not attending the same few elite colleges. I think most of the forum would not consider the majority of colleges on the list to be “elite.” I haven’t seen anything about HLS preferring a particular undergrad college or type of undergrad college, although Harvard College students probably do have a notable advantage in HLS admissions, including special programs for HC like the junior deferral program where HC students can be accepted to HLS under the condition that they must work for at least 2 years first. HLS admissions has talked about the importance of work experience, and roughly 3/4 the HLS class spend some time getting work experience after undergrad. Harvard College also gives more of an advantage to SCEA applicants than most other SCEA schools and has excellent FA for international applicants.
That said, I agree with the earlier posters that there are many outstanding law schools besides Harvard that will offer similar benefits. I’d suggest broadening your outlook.