yep, i'll chime in on this one
forklift as a compactor? nah, like everyone else said, BAD idea.
The only force is the weight of the carriage and forks and gravity, not much force to compact much.
I have a customer that does this but they use a GEHL Telehandler Boom lift with telescopic boom an attachment they had made to mount on the end of it that does the job quite well.
The boom control is a 2 way positive control (up and down) which has the capacity to force downward pressure the same as in the raise direction.
That would be your best option imo ;o)