Funny - I came up with this solution too (mounting the folder to a drive letter) to achieve the effect of selectively collapsing folders.
In searching for previous discussions on the topic, my thoughts were along the lines of specifying 'virtual root folders' within OC. It sounds like you're thinking along the same lines.
- When navigating up and down the tree (but all within the virtual root folder), that folder (and it's parents) would stay collapsed.
- When navigating up through the virtual root folder, the tree would expand as normal (unless there is another virtual root higher up the tree, in which case the tree would expand normally only up to that point).
- When navigating down (deeper) through the virtual root folder it works in reverse - normal folder behaviour, until the virtual root folder is opened, which then collapses all its parents
- The same thing would happen when following a bookmark to a folder that has been tagged as a virtual root folder as suggested by Adriano)
Good luck with all the projects and to do lists!