No one has posted an answer for this question, so I thought I would take a stab at it and remember some of it is more about code intent rather than written law.
First, you have a split bus panel. This panel was developed because of a loophole in the early code that stated you could have 6 means of disconnect to disconnect totally from a service (the 6 disconnect rule). So manufacturers decided that instead of paying for an expensive(?) main breaker, they would instead create a panel with six means of disconnects, diversifying the load and using lower priced smaller breaker for disconnecting purposes. Like I said it was a loophole, which was caught by the NEC and then immediately corrected. That is why there is a very small time line where these panels were used, because they are now illegal.
Now that you have a brief history let's get to your question. You panel was installed when it was legal to do so, and it can stay that way under the grandfather clause. Since it was a generic panel, the installer could have used all of its capacity (6 disconnects) but didn't. You now want to upgrade your panel to its full capacity by adding breakers.
Now here is where the intent of the code affects the answer. When you remove and add breakers to this panel, you are modifying a panel. When you modify a panel you must bring it up to the updated code and if you can't do that with the existing panel, you should replace it. In other words the panel was installed during a time where it was legal to do so, but you can't "grandfather" in an upgrade.
Now I know this could be a big discussion and a lot of it depends on each persons interpretation of the NEC. I will say it really doesn't matter what any ones opinion is, including mine. The only one who's opinion and interpretation counts is the AHJ. That is what they are there for, and that is what they do. Most people hesitate to go the the AHJ because they fear they will rule without considering cost or other installation problems, and they will probably want to inspect the work after it is done. Then again that'is what they are paid to do.
Hope this helps and good luck.