What I'm asking is if there is any loss in fidelity if I take an AE project, render it as a lossless file, then bring the file back into a new AE project.
Rendering to a lossless codec means exactly that. It's lossless. Pixel for pixel the files are identical providing that your color management and Gamma settings are correct. There is no loss in resolution, and no added compression or color artifacts.
What Rick said. Though of course this is only true for 8bpc projects. 16bpc and 32bpc will require to use image sequences, as there is effectively only a handful of CoDecs for clip based formats that support 10-16bit color and none that I know of that is truly float...
What Mylenium and Rick say is right, but there is one important thing to ask when someone says that a codec is "lossless". Some people abuse that term to mean "perceptually lossless"---i.e., a human can't tell the difference. When you think that someone might be misusing the term this way, always ask if they mean "perceptually lossless" (which is actually lossy) or "mathematically lossless".
A couple of examples of mathematically lossless codecs are PNG (at highest quality) and Animation, both of which can be used as video codecs in a QuickTime container.
Why does the distinction for perceptually lossless matter? Because much of the information that computers use for things like motion tracking and color keying is exactly the information that humans can't distinguish---so a perceptually lossless codec can actually wreck a video for compositing purposes.
Ok, new question. Working in CS6 now, and I'm trying to interface with Davinci Resolve, but Resolve doesn't support .avi for some reason. I noticed that AE "lossless" format is a .avi file. Is there a way to export a lossless .mov instead, or is .mov always lossy?
There are a few lossless codecs in the standard QuickTime installation. I prefer the PNG codec in a .mov wrapper, but others prefer Animation. PNG has smaller file sizes at the expense of more computation to encode and decode.