Protecting multiple outlets with one GFCI; is it possible with this wiring scheme?
I take this to mean that there is one main wire that circles the perimeter beneath the floor, and right below each outlet there must be a junction box with a separate wire going up to the receptacle.


This sounds most likely, however, if done to code, junction boxes shouldn't be hidden in inaccessible locations (walls/floors/ceilings that do not have access panels over the box). If there is a floor beneath the floor or an attic above, this is the most likely location of the junction box.

You may be able to find an outlet further upstream which supplies power to the other outlets, but in comparing the price of a combination GFCI/AFCI breaker (~$40) with a single tamper-resistant GFCI outlet GFCI (~$20), I personally wouldn't spend any more time investigating for the additional $20 and would just swap out the breaker.