"How can you convert a Weber (NG) Natural Gas Grill to (LP) Propane? Ideally without manufacturer specific parts?"

Yes, it is possible to convert from natural gas to propane. Having a Weber makes it even easier. Give their customer service department a call at 1-800-446-1071 and they'll be able to tell you if the conversion kit is available for that specific grill. If the grill is old, there may not be a kit. In that instance, you could simply purchase a new grill, or try one of the specialty grill shops that sells replacement parts. Many of the shops that specialize in grills, spas, and the like sell conversion kits that will work with most gas grills (depending on the size). The last time I priced a conversion kit for a Weber, it was 52.00. I hope this answers your question. If you think of anything else, don't hesitate to ask.