I would use Doohickie 's strategy for estimating, but I would change the equipment to work out how much abuse would be possible under worst conditions.

I will use a 200kg (440lb, 1960N)) rider putting all their weight on a 185mm crankarm
Lets use a 20t small cog (smallest commonly available chainring)
and the largest available SS freewheel sprocket: 28 teeth was the largest I found in a 2 second search

T = 1960N X 0.185m X 28t/20t = 507Nm
or
T=440lbs X 185mm/305mm/ft X 28t/20t = 374 ft-lbs