MorganS:
Double-check that your probe isn't overloading the circuit. Does it still function while you're probing it?
Thanks for the suggestion! I changed my setup to allow the controller and DR100 to be connected while probing and sure enough, I'm getting a nice strong 3 volt signal out of the tip of the cable! The recorder functioned as expected while connected to the remote and scope. (This of course makes much more sense than -75mV when I had the tip just going into the probe on the scope)
Regarding LANC, I do have 2 LANC equipped cameras and while dapters are available to use a LANC controller with Tascam products, but they are not directly compatible. I know nothing about how the communication is handled, but LANC runs on 5-8 volts. Thanks for the suggestion though, I had not considered that.
Given that I now have a totally sensible and expected signal coming into the oscope thanks to your suggestion, I'll try recreating this signal at 3.3V and report back with the results.
Here are the new oscope screenshots in case you're curious.
