Does the blowby increase or decrease when you rev up the engine? If you set the oil cap in place without tightening it, is there enough blowby to blow it off the engine?
I agree with taking it apart, or at least taking the head off. Unless the ridge is really deep or there are deep grooves in the sides of the cyl, 0.5mm up from the current size will almost always clean it up.
Make sure whoever does the boring is well versed with VW diesels. The spec piston to cyl clearance of 0.0012 is very tight and many machinists will bore it too large, essentially giving you a block that is already half worn out to the next piston size.
Before taking the pistons out, measure the piston protrusion of all four. Make sure you clean the carbon off the tops of the pistons first as the carbon can easily be enough to go from 3-notch to 1 notch on the gasket. When you get the new pistons, measure the wrist pin to crown height on the old and the new pistons (new German pistons usually move the pin bore up 0.010" with each oversize) you can then tell the machinist how much to take off the head gasket surface of the block to be in spec for the head gasket sizes.