I've been looking around, and is there really no way to do this? I simply feel we're missing something. Has anyone come up with maybe a different way of doing this besides the bulky telescoping tubes?
The easiest way to hide the blade is to hang the lightsaber from your left armpit under your Jedi robe. Then when you draw it you do it as part of an extravagant turn around the spot so that the person in front of you does not see it. It takes a bit of practice. Of course you need a quick release mechanism, but a Covertec clip should be good enough.
One old idea I have never seen realized is the lightsaber hidden in the robe's sleeve: A lightsaber blade cylinder including electronics and batteries slides through an open-ended hilt tube with the help of motorized rubber rollers. A black sock helps hide the blade when it is in the sleeve.
The actual hilt might have to be a bit bulky and of course you would have to keep the arm straight when using it.