LDraw.org allows third party part in the LDraw Parts Library

[Tore Eriksson]

IMO there should be separate library(s) for this stuff in principle. But of course that would not be backwards-compatible with old tools.

The only problem I see with a separate library or folder is that the parts won't be listed by MakeList.
But a unique prefix to the part number, for example t1234.dat or tp1234, would make the files much easier to spot.