This kinda sounds to me as though it is much easier for gliders to have placarded weights for the seats, which presumably come straight off the W & B schedule, than it is for a PA-38 because gliders don't have the variable of fuel, rarely have baggage compartments .... and are most often tandem rather than side by side.
I am not sure whether the OP was asking the structural limits of the seats or just asking what the max pax load was; something that obviously varies with fuel load.
Genghis mentioning 77kg / seat is interesting because I (regretably) come well into the "fat sod" category. Am I taking my life in my hands everytime I get into the 172 ?