From what I've been told by Montana's certifying officer, WHICHE applicants should be put into a seperate catagory as WHICHE canidates. From my experience so far, I don't think that it really helps your chances of getting into a school, although it seems like it should. This makes getting into school from a state that doesn't have a dental school more difficult.
As far as getting certified as a WHICHE applicant, you send an aplication to your state certifying officer. Once certified as a resident you are put into a pool that could possibly get WHICHE funding from your state. Then when you are accepted to a school, they decide if you will get the WHICHE funding from your state.