Control-Click only works for Apple's Menu Bar Extra's, not 3rd party one's, unfortunately. :)
In the old days, a hack was used to make this work by tricking the system into thinking it was a built-in, but unused, menubar item, often the drawing tablet's status item. I believe this was done through a "haxie", from Unsanity. I'm not sure how iStat Menus does it, but I'd hazard a guess at the same way.
Disclaimer: I'm working off shaky knowledge that I learned three or four years ago, before I was even a dev.
Is that what happens now? They used to make the whole windowserver disappear. Not fun.
I honestly can't understand why Apple doesn't improve support for plain NSStatusItems. There's no reason why they can't have command-drag remove and reordering as well. They're just windows, after all.
It might have been the option-click-drag re-arranging or the option-click-drag-to-desktop to remove from the statusbar (or both of those). Maybe those have since become part of the standard API though.