From past UMich law TLSers, your proposed option is definitely be way to go. There's something on the listserv where people post when they want to sell things so maybe once we get our .edus, we'll be able to get news on sublets. It does seem like it's easy to get a sublet at far below market price during the summer and get a jump start at the best apartments for fall when they become available.
However, I'm not a big fan of the whole moving process so I want to get settled at one place where I can be happy (hopefully for all 3 years).
For TLSers who are in the same mindset as I am: I emailed 411 Lofts and their only availability for May to May is rooming in a 3 or 4 bedroom with undergrads. Not really an option. University Towers has a couple May-May availabilities but the photos make the rooms look pretty old and the building isn't very aesthetically pleasing.
I'm on the summer housing hunt as well and will keep you guys posted if I find any good options.