As far as getting free-space info into the tooltips for the icon, I don't think there is another way -unless one patched the filer itself to show such info for mount points.
Personally, I just use gtkdiskfree. It has a dynamic display, which means that it is also polling, but less expensively than a bash script would be.
The problem with using the tooltips is that the AppInfo.xml must be re-written every cycle and could be corrupted while doing so. I think a right-click action (to launch some indicator) would be a better solution.