I concur, external links are an ideal tool for this kind of close integration between projects. I would add a few other ingredients into the mix to really make it shine. The following screenshot is not one project—but two projects that are working together:
On the left side we have our main writing project, which has a fairly recognisable layout: the binder on the left, a corkboard with a column of index cards to the right of that, some text that we are working on, and finally an inspector pane. The inspector here is notable in that it contains a link to a specific piece of research in the research project to the right. If I double-click on the bookmark, the project on the right switches to that file for me (even opening the project if necessary).
How did I get that link? Simple:
- I opened the Bookmarks inspector tab.
- From the project on the right, I dragged the icon of the document from its header bar into the bookmark list in the inspector of the project on the left.
As for the project on the right, in my setup here I actually turned the binder off entirely (View/Hide Binder). Since we can toggle that element on and off easily with the ⌥⌘B shortcut, for this kind of layout it really isn’t necessary to have it always there and in the way. However I could—and seeing as how the space that it would take up is occupied by the space used by the inspector, it wouldn’t even really be that jarring, or cover up my text split on the left.
You might be thinking that you wouldn’t want to have to limit how you work in this research project to a narrow editor view and no binder or inspector! That’s where the Window/Layouts/Manage Layouts... tool comes in. You could in fact save a layout for both of these project windows. Perhaps 65% of the time you don’t need this kind of close integration between the two projects. You are working in one of the other mostly exclusively and would like for it to fill the screen. But when you need constant and parallel access to the contents of the research project while writing, you can quickly access these layouts (check out the “View” toolbar button on the far left) and now everything is all set up, with the research project acting as a subsidiary reference to the primary working project.
Of course this is only one possible way of handling it, using a 15" laptop sized screen. If you have an iMac you might be able to fit more at once and this kind of compact arrangement wouldn’t be necessary. If you have multiple screens then you can spread things out even more.
The core idea is not bound to a particular look, or even an enforced simplification of the project window (although I do kind of like the idea of focussing a project on singular look-ups, when in this role), but rather the simple concept that projects can be tightly integrated with one another, lowering the level of friction in thinking of them as separate projects. Each chunk of text within each scene within each chapter of your novel can have its own separate Bookmark list, meaning its own quick access to the research and background material that relates to it. And with double-click access to whatever you need, the downside of not having that research in another split is—while undeniably still a factor—at least greatly mitigated.
In a way, we can almost think of the research project as being a very elaborate and capable QuickReference window. 
