I've checked the docs and don't know if this is possible. I want to add a toolbar button which points to specific directory (C:\Projects). Clicking this button would reveal a menu listing all the subdirectories in Projects. Clicking on a menu item would make the subdirectory the current folder in the current tab. Possible?
At the moment, no.
You can make a button which goes to C:\Projects, and you could manually make a menu with buttons which go to each sub-folder of C:\Projects, but there's currently no automatic way to create such a menu and keep it up-to-date as the directory changes.
This, or something similar, has been requested a couple of times before so you might see something along these lines in the next version of Opus but I don't think such a feature has been confirmed/announced yet. If you want to increase your chances, drop GPSoft a quick message to let them know what you want.
Thanks nudel, I've done so.
Resurrecting an old thread - is this possible now with DOpus 9 or 10?
Yes, make a menu item Go C:\Projects FOLDERCONTENT (or some variation depending on exactly what you want).
That's great, thanks. Looks like Go "D:\My Documents\My Dropbox" FOLDERCONTENT/O button,nofiles is what I want.
just a comment. you are saving exactly 1.0 mouse click by not using the Favourites (which points to "D:\My Documents\My Dropbox") for your task. Not a big savings imho 
and your button certainly does not open new (=additional) tabs for each time you choose a new current folder (from your menu). the current tab is recycled.
Actually, more than that (the above was just an example). I've actually set up the menu item to go to a sub-sub-folder within Dropbox. This sub-folder has ~15 other subfolders (now appearing on the menu) which I access fairly often but not enough so that each subfolder makes it onto Favourites.