Well one thing to consider with freeform corkboards is that the software would need to save the locations of where you drag cards around. If you select a few things from the binder, then there isn’t a safe place to save that information—the selection itself isn’t something that is persistently stored anywhere, if that makes sense.
But that does bring to mind a tool that is essentially a bit like a saved selection: Collections. As saved objects in your project, collections can store information, such as where cards are placed on a corkboard. So try this:
Select the first folder you wish to work with, and either select its subdocuments manually, or use the Edit ▸ Select ▸ Select with Subdocuments command or keyboard shortcut.
Use the Documents ▸ Add to Collection ▸ New Collection menu command. The collection tab list will appear, and you can type in the name you want to use for this “selection”.
Click on the “Binder” tab to return, and now that you have a collection in the list you can continue selecting stuff and either dragging and dropping them onto the tab in the list, or using the menu command to target this collection.
Once you’ve got everything gathered together that you want to work with, select the collection tab you created. In the header bar for this collection sidebar, you’ll see a few buttons, one of which looks like a hooked arrow. Click that.
This loads the collection itself into the main editor. Now you can switch to Freeform mode, and have at it.
Note to Windows users: the ability to view collections as freeform corkboards is not yet implement.
Once you’re done, you can delete the collection, or just set it aside in case you ever want that arrangement again in the future. And if you’re curious as to how a collection can work as an actual saved selection:
Open the collection, and in the sidebar list hit Ctrl+A / ⌘A to select everything.
Use the Navigate ▸ Reveal in Binder menu command. And there you go—an actual selection in the binder showing all selected items in their original contexts.