<p>Let's say I've made a 3D scan of my face and managed to get that into FreeCAD. How might I then create an object (it's a mask, okay?) that conforms to the shape of my face, with a given thickness, such that I can export and 3D-print that part only?</p>
<p>So if I printed it, it would fit over my face, but still look like my face from the outside, too. It would be, say, 3mm thickness througout.</p>
<p>To be clear, I'm not looking to make a 3D model of my head (the world does not need such a thing), or 3D model of a mold that I can use to replicate my head. I just want to make parts that conform to the shape of other, complex, parts.</p>

It depends on the software you're using, but here is an example with TinkerCad.
Step 1: Import your 3D model (imagine that the ball is a head):
Step 2: Change its type from "Solid" to "Hole"
Step 3: Create a "Solid" box around your imported model.
Here is the inverted Solid/Hole version:
Here is the Solid version with the Hole model inside your Solid box:
Step 4: Select both models and group them:
Step 5: Add a box covering half of your mold (ideally splitting it in half):
Step 6: Duplicate the mold and the box
Step 7: Group the left mold with its surrounding box:
Step 8: Invert the box of the left mold by taking its left corner and dragging it over to the right side of the mold:
Step 9: Group the right box and the mold.
Step 10: You now have two molds for each half of your model:
Based on your comment, I'm adding a couple more steps:
Step 11: Take your cast, duplicate it, enlarge the duplicate, make it a hole and fit it over your cast (like so):
Step 12: Group the left cast with the larger "Hole" copy:
And if you're going to 3D print it and fit it over the old model, then you might want to enlarge the cast by a few mm.
It's now up to you to figure out how to clamp the molds, inject them, and then separate them.
A few things to keep in mind:
You might want to play with the placement of the model inside the mold a little better so it's not at the bottom of the mold but more towards the middle.
You could also get a bit more creative with the joining of the two molds by adding channels or some kind of way that they can fit into each other with greater precision. Here is an example:
.
If the parts are more complex, then the cast will be more challenging and might require to be split into more parts.
Good luck! :)