Actually selective sync doesn't fully handle this case. With selective sync you could choose not to sync down certain files to the computer, but then they aren't accessible at all. With ZumoDrive, the files are accessible but "deflated", meaning they take up no space but will stream and cache on the computer when accessed.
You could do what dropbox did and make a real native UI for each platform. Honestly, I gave up on Zumodrive in part because I couldn't get your menu icon to not look selected. It always was highlighted, even before I clicked on it the first time. Also, the menu behaved in a weird, nonstandard way.
Zumodrive actually supports sharing either by a public link or to a private email address. There is a share option in the right click context menu as well as on the web file browser.
