Paul, I'm sure you'll find plenty to photograph in Rome. I know you've asked for spots that are "off the beaten path," but the reality is that all of Centro Storico is beaten path. That being said, if you're starting from near the Colosseum, I would recommend that you walk up and into Parco di Traino. There are some magnificent perspectives of the Colosseum from there, particularly in morning and evening. If there's a concert planned in the Colosseum while you're there the lighting inside will be particularly special and could warrant some late evening photography.
I would also buy a pass into the ruins of the Roman Forum (it's included in the ticket price to enter the Colosseum). There's plenty to photograph in there and if you go during midweek it's usually not completely swamped with tourists. At the end of the forum pathways there are a bunch of steps that lead up to the left backside of the wedding cake building (Altare della Patria). Go up the steps, then down the long stairways on the other side of that little piazza, then to the right and into the main entrance of the Altare della Patria. Everyone and their mother goes to see this, but many do not have the patience to go all the way up the building to the upper left back corner. From there you get great views of Via dei Fori Imperiali, the forum and the Colosseum.
From there, it's really all up to you. If you want to keep walking, then I personally would recommend walking into the Jewish quarter, toward Fontana della Tartaruga, then further toward Teatro di Pompeo, then further toward Campo de' Fiori. I lived in Campo de' Fiori for four years, and it's an amazing place. It constantly changes character throughout the day. Open air market from dawn to 1. Then the market clean up. Then tourists and locals enjoying lunch, then the aperitivo, then dinner, street entertainers and the bars filling up, then (on a packed night) total chaos when the bars close at 2. This may or may not interest you, but there are many opportunities for interesting medium format shots. I think your large format work will be much more easily done in the early spots. B.t.w., if you need photo gear while you're there, there is a place right off of Piazza Campo de' Fiori (across from Palazzo della Cancelleria) which has a lot of analog gear. I think they're a snotty lot, personally, but they're your best bet in that area.
Now, if you have the energy of an 18 year old, you could continue further. I would recommend going from there into Trastevere and just walking around, then walking up into the Gianicolo (the hills above the Vatican).
Three words of advice:
First, you will get lost. You will likely get very lost. Get yourself a good map and pay attention to it. Enjoy being lost though, that's when you'll see the best stuff.
Second, watch out for thieves. I have witnessed many many snatch and grabs, particularly by gypsy children. Please understand that I have no desire to offend anyone who may be Roma; I think its an intriguing culture. Notwithstanding that, I have personally seen many gypsy children committing petty crimes in Rome. Your camera gear, especially if it's an easily grabbed backpack, is easy pickings.
Third, eat dinner at Da Nerone while you're there. It's right near the Colosseum and it's got very authentic hearty Roman food. Make sure you get fiori di zucca as an appetizer. mmmmmmmmmm. The address is Via delle Terme di Tito, 96 Roma.
If you have any questions, PM me. Enjoy.
