First thing to establish is what material the pipes are made of.
If they are the old Pitch Fibre type the only solution is to replace them. They were introduced in the 1950/60's as a 'new' cheap material that subsequently was found to flatten/squash and disintegrate over time and so were discontinued.
If they are Glazed stone pipes or Plastic (if an old installation unlikely) then if you can scource some Hydrochloric Acid, pour it into the drain at the beginning of the fat problem, it will dissolve ANY organic material. However, it is a dangerous substance, take great care not to come into contact with the acid or inhale any fumes given off, also be aware the reaction can be violent when the acid comes into contact with organic material!
Hydrochloric Acid is used for several purposes, it comes in plastic containers as a dilute solution. If you are a genuine business you can buy it from chemical companies such as those that sell cleaning products to commerce, alternatively you could contact a steam pressure washer supplier/servicer who use it to descale their washers and buy it from them.