the only schools that I have ever definitely heard of doing this (ie have some people start in aug/sept and some people start around jan) are some of the carribbean schools.......its not something that I had researched....so their may be some US ones that do this...not sure....but if there are some I would assume its very very few...
UMKC, which has a 6 year BS-MD program, usually has a few openings each year for MD-only due to drop outs midway through the BS-MD program. Entry into their MD-only pathway is January.