Now I don't know when wireless radio triggering flashes became ubiquitous, but I doubt it was available in the Nikon SP era. I bought a trigger and four receivers last month on eBay and have been really enjoying using them with my DSLR for commercial shoots. Since the SP has a metal shoe it interferes with the trigger's connection, so I used a small piece of card stock to block the connection. After that, simply connecting the PC cord to the flash sync terminal on the SP was all it took to slave my modern flashes to the SP!
To test out both this setup and an idea I had for setting up flashes in a church, I took photos of a group I work with often called Dominion and Power. They are a gospel group that performs around South GA very often. Anyway, the setup was two flashes on tall stands (about 12' up) on either side of the church with an umbrella to spread and soften the light. My flash meter still read f/3.5 with the flashes at max output - but the church was pretty big so I considered that a pretty good result. In the future I'd like to get a couple of double flash brackets and slave four flashes in the same configuration, which would at minimum allow me to shoot at f/5.6 or so. Either way f/3.5 was plenty for DOF considerations on 35mm (I was mainly concerned about cutting the ambient light since the SP only syncs to 1/60).
Here are some results - with either Tri-X shot at 200 and developed in Rodinal 1:100 semi-stand or Fuji Superia 200 color film:
In this one you can see my flash to the left, so you get an idea of the setup:
Now that's not the only camera I tried this with - I did take just a few sheets with my Polaroid 900 and Xenotar 135 f/3.5 using the trigger system. You might remember I metered at f/3.5 - which is all I could do with the Xenotar! So the DOF was nil. I nailed a couple of the shots, but here are my two favorites. The b&w image was with Tri-X 320 shot at 200 and stand-developed in Rodinal 1:100 and the color shot is Portra 400NC overexposed one stop (it's quite old so that was a necessity):
Now I shoot these kinds of shots all the time with my digital - so what's the point?? That's a great question, and one I've been pondering on quite a bit. What I see is that the 4x5 images are not only vastly superior in quality, the negligible DOF is quite engaging and very beautiful. I would love to shoot some commercial work with just 4x5 if possible (and the client is willing to pay my nominal rate PLUS film/dev costs!). The 35mm is just as obviously not superior in quality to the DSLR - but they are still quite engaging. I notice that the greens and especially reds are much more vibrant than what I get with digital (digital Bayer-sensor type cameras have trouble with red especially). That said, with a bit more light, or a faster shutter speed along with a lower f/stop, should allow me to shoot some Portra 160 or Provia instead, which will likely be much better than the Superia. Of course, I could also shoot some medium format - my Bessa RF should work nicely. So a lot more to try with this type of shooting.