Hi Sue, as promised the following is some data retrieved from my collected xeroxes of material pertaining to the Campo Marzio, which may be useful to your present work regarding Piranesi and the Pantheon.
1. Samuel Ball Platner in The Topography and Monuments of Ancient Rome (1904) gives a concise description of the:
Basilica Matidiae, basilica Marcianae. These two basilicas were between the Pantheon, the north end of the Saepta, and the column of Aurelius. One of them was named from Matidia, the mother-in-law of Hadrian, and the other from Marciania, the sister of Trajan. They probably formed one group with the temple of Hadrian. Some cipollino columns that have been found just north of the via dei Pastini, between the Pantheon and the vicolo della Spada d' Orlando, undoubtedly belong to one of them.
2. The book on Hadrian I mentioned is:
Anthony R. Birley, Hadrian: The Restless Emperor (NY: Routledge, 1997). Chapter Two, "The Campus Martius" is all about Hadrian's architecture/building activity within the Campo. One illustration is of a reconstruction of the "South Building" once attached(?) to the south of the Pantheon. The plan of this building matches the 'Xystus' Piranesi delineates within the Ichnographia. The "South Building" is often referred to as the Basilica Neptuni, and, for the record, Piranesi positioned the Basilica Neptuni somewhere else within the Ichnographia, that is, just east of the large sundail.
3. According to Freund's Latin Dictionary:
- among the Greeks, a covered portico or gallery, where athletes exercised in winter
- among the Romans, an open colonnade or portico, or a walk planted with trees, etc., for recreation, conversation, philosophical discussion, etc.
4. I made a cursory overview of Aitken's thesis and I found no direct mention/analysis of the Pantheon complex within the Ichnographia, nor did there seem to be any mention within Aitken's treatment of the Campo Marzio text that Piranesi intended to publish a separate volume on the Pantheon.
Re: research assistance
The Basilica Neptuni seems to be a building that Piranesi 'played' with within the Ichnographia. According to Nash's Pictorial Dictionary, Palladio drew a plan of the Basilica Neptuni (as illustrated therein), and Piranesi obviously knew this plan because he used it [and labeled it Xystus] within the Ichnographia. In my experience, whenever Piranesi makes what seems to be an obvious mistake within the Ichnographia, that's usually a sign that Piranesi is playing a(n inversion) game. That may not be the case here, but you should at least be aware of the (intentional?) transposition of buildings.
Platner also has some to say about the Basilica Neptuni, as I'm sure other sources do as well.
From the 'Catalogo' of the Campo Marzio:
Basilica di Nettuno, «El Sparzian. in Adriano» Vegessi Portico di Nettuno. --I assume this is a reference to the Hadrian biography within the Historia Augustus.