As you point out many states have no additional requirements for MD's to practice acupuncture and others have very limited requirements. This is done in large part because the types of conditions that MD's would actually treat in a medical setting are limited to those for which there has been sufficient research on the ways and means of treating that condition. In other words, they are licensed, trained and operate under the premise that acupuncture is a reproducible science that anyone can do if they simply follow a treatment protocol and insert the needles into well defined locations.
It is very rare that MD's are able (or even try) to treat the full range of conditions for which acupuncture can be used - which is essentially every condition that is not immediately life threatening. To be able to treat a full range of conditions (i.e. generally not just pain cases) you will need to be fully trained in all of the underlying theories and treatment methods of Chinese Medicine. Personally I am not certain that you would need to go to an acupuncture college, which as you point out, would be another many years in school covering many things you know in more depth already (for those that don't know about 1/3 of most acupuncture college curriculums cover western medicine). You should, of course, take one of the many acupuncture programs for MD's just to get the basics and to cover any licensing barriers to your ability to practice. The American Board of Medical Acupuncture has a list of approved programs that you will want to read over.
After your initial training, if you are really interested in practicing acupuncture in its fullest expression you will need to apprentice very closely with someone (or, ideally, a few people) for a year or two. This experience coupled with reading some of the foundational texts should get you started on your lifetime of becoming an acupuncturist. In the martial arts, there are people who are good and those not so good, but no one is ever finished learning - acupuncture is the same way.
Even people who attend a full acupuncture college still benefit greatly from the practitioners they associate with both after their formal schooling and throughout their lifetime. College's teach the basics, enough to treat a range of conditions well - but only time, further thought, and an open mind will make you a good practitioner. I say this only to encourage you that with proper guidance and enough interest you will become an acupuncturist.
Yin Yang House Acupuncture & Energy Healing Clinic