Can a MEI CFI give a valid flight review in a plane for which he is not endorsed?
The flight instructor is limited to give training in the category and class ratings of their pilot and instructor certificates, not by make and model.

Per §61.193 and §61.195 the flight instructor is authorized to train and issue endorsements for a flight review so long she or he holds a pilot certificate and flight instructor certificate with the applicable category and class rating.

Furthermore, per §61.51(e)(3), the flight instructor may log pilot in command flight time for all flight time while serving as the authorized instructor in an operation if the instructor is rated to act as pilot in command of that aircraft.

The instructor is advised according to Advisory Circular: AC 61-98C,  Section 4-2 Paragraph e. Instructor Qualifications:


  Instructors should also consider their own experience and
  qualifications in a given make and model aircraft prior to giving a review in that model... ...To conduct a flight review in a multiengine
  airplane, the instructor must hold an airplane multiengine rating on their pilot and flight
  instructor certificates. For aircraft in which the CFI is not current or with which he or she is not
  familiar, he or she must obtain recent flight experience or sufficient knowledge of aircraft
  limitations, characteristics, and performance before conducting the review. In any case, the CFI
  must observe the rating limitations of § 61.195.


So, yes.  An appropriately rated flight instructor (whether or not they should) is authorized to conduct the flight review.

What's more, even if the instructor is not qualified to act as PIC (e.g. does not have a high-performance training endorsement), that instructor will still log pilot-in-command time as instructor because he or she is appropriately rated to do so.