How to remove copper from inner planes on cut PCB
As copper is very malleable, grinding is probably not going to work very well.  Sanding is better, however it can also leave copper fragments that could pose a problem for you.  My best guess is some amount of etchant + sanding will be your best option.  

I'd also seriously consider the total labor costs of your mods compared to making (or having someone else make it for you) your own pcb.  Depending on the complexity &amp; component costs, that may not be a feasible option--especially for only 150 units.

Best regards