In the fly-out menu of the Pages window, you can select Load Master Pages, and when you navigate to a document that contains the master pages you want to load, you can transfer them to the document you have open. If the destination document already has a master page with the same name as the master page in the source document, you can choose whether it replaces or is added to the master in the destination document. In your case, you would likely want to replace. If you keep a one document that will serve as a "master," you can update it's master-page items when you need, and load those master pages into your other documents as described above. If you ask over in the scripting forum, they might be able to tell you if this can be automated to do several documents at once.
I just realised there's a Save As Template option, but I'm not quite sure how that differs from the standard Indesign file.
A template is just an InDesign file that opens differently. Instead of opening normally, it opens as an untitled copy. Anything you save won't save over the template file, unless you give it the same name and save it in the same location as a template. The advantage to using a template instead of just reusing a normal document over and over is that a reused document could develop a bug over time, and a new document is less likely to do that. So it's really just like opening a new document, except that it can contain master pages, styles, images, type…basically anything you want to use that you don't want to recreate each time.
I've put a post in the scripting forum now.