Dave,
If it's "rock solid" it's not laytex, which remains rubbery.
How does it respond to attack with a perforator?
Solid masses become more responsive to solvents when fragmented and if a few small sections can be removed they can be tested with solvents away from the carpet,
If you have a steam generator, see if it responds to heat.
If innitial investigations do not indicate a relatively easy method of removal, especialy since there is a posibility that this contaminant has penetrated to the backing or beyond, I would sugest that this may be a job best walked away from whilst it is the customers' problem - not yours.
regards,
John.