I have a Smart folder showing my entire hierarchy of Clients | Projects | Tasks | etc.
It would be useful if I could “Favorite” any part(s) of that hierarchy – e.g., If there are only a few specific Projects that I want quick access to, it would be useful to have just those parts of the Smart Folder reproduced in my Favorites (along with the hierarchy under them).
It also seems not currently possible to Favorite an entire Smart Folder – or any Folder.
That is also something I’d find useful.
Have you thought about adding a ‘Favourite’ checkbox field to your Projects database, and then using a filter on the smart folder configuration to only show those that are checked?
Smart Folder filters appear to only be “global”, so I don’t think that would work.
In that case, why not create a new field, called ‘Show this project for’ which is a many-to-many relation to users, and if it’s a Project you want to see, add yourself to this field. Then configure the smart folder to only show Projects where this field ‘contains Me’.
You would probably actually want to set all Projects to include all users to begin with (using an automation on creation perhaps, plus manually for existing projects) and then remove yourself from those you don’t want to see.
I know I could do all that, but I still think it would be a neat feature (and much easier) to simply be able to transpose part of a Smart Folder – or any Folder from the Left Menu – to your Favorites area 
Thanks for the tips and this post! 
I wanted to favirote some “hub” items in a smart folder for quick items, but I didn’t want to favorite each one, since it would take to much effort and take to much space from the favorites area.
From the discussion in this post, I realized I could instead create a relation to User, and create a View that filtered by the “Favorited by” field, which neatly my problem. It required manual work (more than hitting a Favorite icon) but Fibery made it easy to setup, took a minute.
I still need an “extra” click (first access View > then Hub item within View) compared to having the full navigation in the left sidebar (i.e. like how folder and smart folders work), but on the upside, the View is purpose-tailored and flexible.