I'm going to answer my own question, but I'd like to see opinions on whether folks think this might be the answer.
So in this scenario, the rendering that is my sub-layout has no actual data going to it; it's purely a placeholder deal. Think a main layout that has a Bootstrap col-md-12, and I want a sub-layout that has three col-md-4's, with a placeholder in each. So I create the rendering in Sitecore, connect to a CSHTML file with the markup, and in theory I'm done except for "where do I put the placeholder settings?"
So on the layout page connected to the template, we have this:
The placeholder settings for "main" can be assigned as the default key on the placeholder setting itself, as discussed below. Then we have a rendering that consists of the following, with no template required because it's just placeholders:
Each of these placeholders would use the same placeholder setting, let's call it "Single Column". So the question is, where do I assign "Single Column" to affect these three placeholders? With the caveat that this rendering may not be used, and I'd prefer not to junk up my page template with placeholder settings that may never be used.
I can't put them on the rendering, that doesn't work correctly. So after spelling the case out above, the thought that came to me is to create a template with no fields, and all it has on it are placeholder settings. I then attach that template to my rendering in the "Data Source" field, so they're always connected.
In theory this would work. The one thought that occurs to me as I write this, in the standard values of the template to specify the presentation details, can you specify just the placeholder settings, or does it force you to pick a layout and perhaps a rendering as well? If so, could you create a completely blank layout (I mean a CSHTML file that's totally empty, no markup at all) and then pick some dummy rendering that's connected to an unused placeholder key (so it could never render on the page) just to trick the system?
It sounds like a long way around to achieve the effect, but the ultimate goal is, for these renderings that are nothing more than sub-layouts that contain placeholders, to keep the placeholder settings with the sub-layout, and not "junk up" the page template with placeholder settings/keys it might never use.