Rigger 3 does address this.
Seats come in two varieties - Bench and Bucket. A bench seat can be normal or reinforced as can a bucket seat.
Max weight for a standard bucket seat: 100 kgs
Max weight for a standard bench seat: 200 kgs
Max weight for a reinforced bucket seat is 250 kgs
Max weight for a reinforced bench seat is 300 kgs
For REALLY heavy bastards, super-reinforced bucket seats are also available. They start as standard bucket seats but cost 100 more nuyen per 25 kgs they hold.
Costs -
Standard bucket: 700
Standard bench: 750
Reinforced bucket: 1,500+100 per 25 kgs above 250
Reinforced bench: 1,500
All motorcycle seats are bucket seats.
You need to remember something - The rules for how seats work in SR is stupid. See, they aren't designed to ever be empty. What they should have done is give us the weights for the seats themselves and the max weight they can safely hold. Then you just subtract from the Load of the vehicle the weight of anyone that happens to be in the car.
The way it works now is this: When you add a seat to a car, it immediately takes up as much Load as it can hypothetically hold. Example, if you install a bucket seat that is designed to hold 500 kgs, you immediately subtract 500 from the Load.
Edit: To be totally clear, the Load rating of a vehicle already assumes all of the seats will be filled. If your troll tries to fit in a seat that's too small for him, he gets a +2 TN to his resistance modifier during a crash test. He'll also ruin the seat for anyone else that tries to sit in it. If the (max weight of the seat)+(Load*2) is less than however much your troll weighs, no the bike can't move at all. Be sure to read
THIS link regarding the height and weights of metahumans.