If I understand you correctly, you could, in the Text tool :

1) create all the needed text inserts in the score (at the size you want, and set their parameters such as positioning  top, bottom, centered or aligned, page range for instance)

2) For those texts, right click on their handle and check Unlink in all parts

3) In the score uncheck Show on all these texts that should only appear on the parts.

As a result, these texts will only display on the parts.

Thanks for your suggestion, however, I don't see how that is any simpler than just using the text tool in a linked part. Seems like a couple of additional steps, actually. I've added text items in linked parts, and they populate in other parts. 

Sure, you can add these texts in a part, they will appear in all parts, but ... also in the score, which you may not want in some cases (texts in another size or position from page 2 in the parts, for instance).

If you want to hide some texts in the score, there are 2 more steps to do  :

• Right click on the handle of the texts (you can select several handles at a time) and check Unlink in all parts

• Uncheck Show

In this scenario, the number of steps is the same if you start from a part or from the score.

... where do I set the style and location what I'd call master-page items, things that I want to appear on all pages of all parts?"

You need to check out Frame Attributes. Go to the Text tab then Frame Attributes. You can then use the Show/Hide button as needed. 

Thanks to both Michel and Ernest. I'll give both answers methods a go and see what works best for me.