OK, I'll chime in. Although probably not as you'd hope. I can offer no further information regarding this bullet. As you've correctly stated, most Trans-Mississippi bullets are very rare and little is known about them. If these have indeed all been recovered in the same, single area is it known which troops were in that area. That would be information to persue.
Regarding the name, I think to name a bullet after the guy who first found it is doing a very serious disservice to future collectors. Please understand that I don't know Mr. McCallum, and mean no disrespect to him or his talents as a relic hunter. But his name in no way relates to the use, manufacture, or anything else historically tied to this bullet. This name can only lead future researchers who may never know its origin down the wrong rabbit hole.
Having said that, I cannot offer a better name since there is no information. Perhaps just a "Port Hudson .58" for the time being. Or a "Cleburne II". They are essentially the same bullet with just a slightly different profile.