Dear Mamerca,
I have to admint I had to think a bit about your post before answering and found out that what striked me - maybe becuase of their assonance to Italian - were Cicero's words "illam insepultam sepulturam effecerat..." in a less-than-avarage translation "thy buried those (ashes) which were previously unburied".
This - IMHO - fits very well with your definition of "bustum".
Personally I don't see a rason why Romans would move the ashes after the pyre they had erected in the forum, though... there's no reason why they shouldn't have.
In the same way, I always found it a kind of "missing" info that few sources refer to what happened to JCs ashes, given the importance they were given in ancient Roman society. To myself, I always tried to explain it through missing fragments or maybe to something that would have been obvious to Romans, but it's not such to us, moderns.
For what I remember ashes were usually collectd in urns, so it would be natural to think of JC's ashes having the same fate (wherever they were buried), but... wouldn't that urn have been found by now in the forum where extensive excavation has being going on for centuries? That' just a question from a non-digger...
I'd personally love to see Caesar's ashes found... think of a ADN exam on those ashes...
On the other side, I always found striking that no mention survived of his actual burial position. For Romans it was very important to be able to do offerings on their loved-ones graves...
How can such an omission be justified in the case of a person like JC, whom fonts agree was loved by the people? I don't think it realistic that the few days chaos, after JC's death and funeral, can have canceled in people's mind the exact place where he was buried...
So I ask you . knowledgeable fellows - are we simply dealing with missing sources or is it us, moderns, who are missing something that would have been pleanly clear to ancient Romans?
Just wondering...

P.S.: on a personal ground, I find it easier to believe contemporary sources than later ones...