[EE] Intentionally delaminating a PCB

Hi,

I'm in discussion with a company who had a product developed by another 
entity, but that other entity left the project (to develop their own 
competitive product) and won't deliver final files.
The board had gone through several iterations to deal with noise and 
heat, and at this point they have the schematic, BOM and physical 
boards, but no PCB layout files, nor Gerbers, so they can't produce more.
That's where I come in... they are asking me to reverse-engineer this to 
get back the gerbers so they can continue production. Yes, I'll verify 
it's legit.

The boards are at least 4 layers, so not simple to reverse-engineer.

What techniques are commonly used to do this? I can start by removing 
the soldermask, and then carefully grind away the fiberglass, but 
there's got to be a better way... is there? Any chemicals to eat 
fiberglass and leave copper? Or any way to use heat etc to intentionally 
delaminate the layers so they effectively just peel off? Or any other 
tools to assist with this?

Cheers,
-Neil