I don't see why you couldn't have the best of both worlds...
I believe the 3.4.1 codebase uses some sort of 'commonness' value, where if the commonness of a dagger is 30 and of a blade of chaos is 1, then a dagger is 30 times more common than a blade of chaos. If I am wrong, ignore the rest of my message. ;)
Let the sum of the commonesses of objects be O and let the sum of commonnesses of artifacts be A. If we want there to be exactly a 1/400 chance of generating an artifact, we could scale all of the object commonnesses by (399*A)/O and then do a object generation as Derakon suggests. This would lead to artifacts being generated 1/400 of the time, and in addition we can easily calculate the relative rarity of any two objects or artifacts.
How do you intend to handle this on a level by level basis? Would you basically have 101 allocation tables with commonnesses of every object/artifact on a level by level basis?
Dave
