I think it's widely understood that this is something that takes place, and has been taking place on standard mechs ever since the invasion (see Archer Christofori's Penetrator, the Avanti's Angels, every merc unit that has ever hired Battle Magic, and so on) but not in a standardized way. So sure, probably a third of the Avatars out there have a Clan tech weapon mounted, but its not the same one on any of them.
Part, to my mind, of having a set configuration is that it is standardized. All the quartermasters know it, all units with the mech stock the pods (if they can and want) and it's been tested and approved and so on. The only military until the post Republic with that sort of capability was the DCMS right after Bulldog, which absolutely had several standard Clan configurations (I always assumed R was for "Dragon Roars"). The FedCom didn't then have that many omnis, nor any other military, and most of them probably used most of their salvage either to keep their few Clan made mechs in service, or to refit conventional mechs.
Now, in the Dark Age I would expect to see more standard Clan or mixed tech configurations. But, I guess that either anyone who can afford a Clan weapons suite can afford the whole Clan mech, or else no one really wanted to muddy the waters.
But, in any era, I'd expect custom configurations to be as common as standard ones, and Clan weapons to be as common or more on them than on standard mechs (we're explicitly told that many IS omnis are designed to accept Clan pods right off the shelf). But since no two are the same, we don't get a record sheet.