Well for Temple I know it's true, at least from a relative standpoint. It doesn't directly say you have to be declined but it does say you have to have applied to Temple's Dental program. In general, however, you do not have to be declined. You can do this after you have finished your undergrad to boost your overall/science GPA.
"Applicants who have applied to the Kornberg School of Dentistry Pre-doctoral Program through the American Association of Dental Schools Application Service (AADSAS) are eligible to be considered for this program. Applicants must also meet the following requirements for Post Baccalaureate Program application consideration"
And yes for most you have to have taken your DAT. There could be exceptions though, but from that ones I have seen a DAT is a minimum requirement.