If I have something in my scene that I want rather close to pure black - can I spot meter off the object and then adjust the exposure by a consistent, pre-defined number of stops to get black instead of gray? I'm aware the rest of the scene may be off, but if my goal is that object - is it possible?
Yes. Spot metering gives you 18% grey. Each stop below that halves the reflectance.
The number of stops to dial-in depends on how close to black you want the result and the dynamic range of your camera.
With a perfect noiseless exposure, -1 EV would give you 9%, -2 would give 4.5%, -3 would be 2.25%, etc. As you can imagine, most cameras are not perfect and there is such thing as a noise-floor. If you exposure below this, then all details get drowned by noise.
The best thing to do it to experiment and figure out the value of your camera. Unless you are shooting RAW, the number of stops depend on image parameters, mostly Contrast and High/Low key if your camera has them. For fixed image parameters and ISO, your that value will be fixed for your camera.
Most Olympus DSRLs and SLDs have a metering mode called Spot Shadow which is designed to exposure something very dark while retaining details. It is equivalent to normal Spot metering -3 EV. Doing the same using EC with your camera is probably what you want.
Yes, it's possible. The exact adjustment will depend on exactly what kind of "black" you want, and on the contrast range of the sensor/film you're using. If you want a dead black with no detail left at all, you're typically looking for at least -3 stop exposure compensation. Around -2 will normally give you a "black" that looks quite dark, but still retains some detail, so (for example) you'll typically get a pretty decent impression of surface texture.
That does depend on the sensor/film as well though -- high-contrast film like Velvia can do with less compensation. Lower-contrast film tends to need more. With digital, it'll depend largely on how you process the picture. Especially if you shoot in raw mode, you can do quite a bit of adjustment after the fact. You'll probably want to avoid using any "auto" exposure adjustment that most raw processors provide, or they may adjust things so the part you've metered to be black really isn't any more.