Although it is true that schools say they interview into March/April, if you think about it, they send out invites for those slots early (like in January/February). The actual interview doesn't even occur until about 3-4 weeks after the invite. So yes, they say they interview, but those slots have already been filled by those that were invited earlier.
As for a letter of interest or a letter of intent, I think right now is probably the best time to do it. Personally for me, I wrote a combination of interest and intent. What I said was how the school was the best match for me. Why I thought I would be a good fit and try to hint to them that you plan on staying in the area. Schools like to keep their physicians in the area. For the update, I included by transcript with it as well as what I've been doing since I applied (I was done around August). But I think the emphasis has to be on why you think the school is a good match. Don't just go to their site and list off everything they already know like, "your curriculum is integrated so I think I would learn best" or "you have a lot of clubs and I like clubs" etc... The best way is to relate a personal experience, make it sound genuine. At this stage of the game, letting the school know that you're still interested, whether it be pre/post interview wouldn't hurt.