I understand that as Catholics we genuflect before entering and upon exiting the Church pew as a sign of reverence to the blessed sacrament, but what is a Catholic to do when attending a Protestant church service? Is it proper to still genuflect knowing there is no tabernacle?
Most Protestants do not genuflect. If you should enter one where they do (such as an Anglican church), you would not. Such an act of adoration would not be appropriate for a Catholic since the Catholic Church does not recognize their ordinations (and therefore most of their sacraments) as valid.
The Easter Orthodox Churches do have valid ordinations and it is appropriate for a Catholic to bow (they do not genuflect) in their churches.
Fr. Vincent Serpa, O.P.