A surprising discovery from seeing what happened inside was that a major potential cause of vapour lock was a fuel pump as the suction side could pull the fuel below its vapour pressure allowing vapour to form in the line
Shouldn't have been surprising. Piper surely were well aware that they need to ensure a net positive suction head (NPSH) across all reasonably foreseen operating criteria (ie atmospheric pressure, altitude, temperature, fuel flow rate etc). A simple pressure gauge on the suction side of the pump and then a relatively straightforward calculation could tell them that. I suspect therefore that they knew or suspected they had a problem and were trying to identify the source.
Transparent piping may have helped locate source of any specific trouble spots in the system visually, but a far better way would have been to install a series of pressure sensors at various points in the system and look for any significant pressure differentials.
Increased pipe bore and reduction of bends/joints etc are indeed straightforward ways to improve NPSH, so I don't think your memory is failing quite yet DV
They both work to reduce friction/turbulence thus reducing resistance to flow. If fuel heating (thus increasing fuel vapour pressure and reducing NPSH) is an issue, insulation will help so above advice regarding foil etc would also help, subject of course to approval from appropriate authority in the case of aircraft!