as far as i know, the FAA could care less if you are in the front,back,left or right seat of an airplane. as long as you are competent to fly from the seat you are in. as long as the aircraft does not specify PIC seating, it matters not. if solo in a tandem aircraft it MUST be flown from the placarded seat. as most are aware,CUBS "solo from rear seat only". as for the insurance, if you were listed on his policy,or meet the open pilot warrenty of his policy,and could show compentency flying from the back seat then the aircraft owner would be covered. most if not all policies reserve the right to subrogate the PIC,if they are not listed on the policy,even if they meet OPW requirements. i would not sweat a ramp check either. as long as the proper documents are on board,and on your person,i find it hard to believe an inspector would hassle you. (i would suggest you not mention the part about playing the roll of an instructor) as CFII-MEI with advanced ground ratings, i can't remember ever seeing an FAR that specifies seating arrangements for a 2 place GA aircraft. if you were the only pilot current in the aircraft at the time this flight took place, or for the time you were the "sole manipulater of the controls" it can be logged as PIC. one thing i have done several times for the guys that like to fly both front and rear, is to check them out in the back seat.(trust me,they earn the endorsement) then place an endorsement in their logbooks that reads"flight checked,and found compentent to act as PIC from the rear seat in a (type goes here). don't know if that is necessary, but it at least shows the FAA and the ins or anyone else who might care, that they took the time to get the training.