While I would LOVE to be able to do that, where the category is assigned later on down the chain, it's not possible. Not with a code rewrite, or anything like that. That has to do with the database structure itself. Each subdomain has, for all intents and purposes, a mini db. At the time this split was made, it was necessary due to OS limitations for database size. We were able to trick it by using the tables as mini dbs, in to seeing the different table sets as mini dbs, which made it so we didn't exceed the (at the time) 2GB database size limitation.
Now, many years later, this database is 9GB in size altogether. There is no feasible way to merge all those tables in to one set, and make one database to be able to do what you want. Not even with cloning. We already HAVE a mess we're fixing, and frankly, I am not at all interested in making it even worse and having that much more to manually repair. Me, myself, and I, that is.
If we were to "start from scratch", we'd lose a huge amount of data. That is not acceptable, so won't be happening.
The software itself is actually in the process being completely rewritten and modernized, as we are well aware of the code deficiencies in the old software. However, the rewrite is being done in such a way as to PRESERVE all the data, not lose any of it.