Gavin, I tried to check out this feature, but didn't have much luck. The one image of the Farnese globe I found in which this figure was clearly visible was in Dennis W. Duke, "Analysis of the Farnese Globe" :
http://people.sc.fsu.edu/~dduke/farnese4.pdf It is also discussed in B. E. Schaefer, ?The epoch of the constellations on the Farnese atlas and their origin in Hipparchus? lost catalogue?,
Journal for the History of Astronomy, xxxvi (2005) 167? 196. These two articles debate whether Hipparchus could have been the source of astronomical data for the Atlas figure bearing the globe, which seems to date from 180BC to 70 BC.
According to William Smith's
Dictionary of Greek and Roman Antiquities, one classical source on the Throne of Caesar as a constellation is Pliny. He mentioned 72 constellations in all, apparently by virtue of breaking down some constellations into their constituent parts. Unfortunately I couldn't find this in Pliny's
Natural History, which I guessed was the source. (23AD-79AD)
As you know, the Farnese globe doesn't show stars but naturalistic figures, so it is likely that any stars bright enough in the heavens to attract attention were subsequently assimilated to another adjacent constellation. The rectangle itself is hard to identify with anything. Also, it is possible that the sculptor did not have a thorough knowledge of the constellations, but used some artistic license.
The comet in question was observed in 44 BC, and was well described by classical authors at the time. So if the rectangular feature near Cancer was called Caesar's Throne, it would seem that it received this name after the statue was made.
The comet was seen shortly after the death of Julius Caesar, and his nephew Octavian supported the idea that the comet was Caesar's spirit ascending to heaven as a god. There is some debate about the days of the comet's visibility--whether it would have been in July or September.
One title on the comet is:
Ramsey, John T. and A. Lewis Licht (1997),
The Comet of 44 BC and Caesar's Funeral Games, Scholars Press (Series: APA American Classical Studies, No. 39.)