Why is it next to impossible to identify schools in a given area that use DI
either for certain subjects, or as a whole-school model?
In all likelihood there is no "master list" of DI schools. A few months ago ADI was soliciting the members of the DI Listserv to send in which schools where they were working to create such a list. You might want to contact ADI again to see if that effort was successful.
To find out about whole school implementations you might want to companies that do them: Nifdi and J/P Associates (There may be one more).
There are also a few private schools using DI I've seen mentioned on the DI Listserv.
Other places I've seen DI schools mentioned is in DI News. If you join ADI for $40 a year, you get both DI News and The Journal of Direct Instruction.
Not sure how much this helps, but it's all I have.