Hey, thanks for your input.
First: a glissando "chart" for the clarinet is bogus and useless; just write the notes you would like the performer to slide to and from and they will do it. If it is over the break, they can compensate with quick chromatic and/or lip adjustments. If the performer can not do this, find a better performer.
No I don't think is that useless, Lip gliss can be done an all notes, like in Sax, but would be only -1 whole step at most.
Second: your range chart is off. A Bb clarinet's lowest sounding concert pitch is a D below the E displayed above. What you have shown is the "written pitch," which doesn't change between clarinets (the actual sounding pitch does change).
Ops, yes, my mistake

, I'm fixing that "Written pitch"
Third: A good performer could easily extend the range of the gliss to a high A or Bb (again with the use of the embouchure).
I'd say G is the fair end, altissimo skills should not be included on ranges, is like Sax, F# is the end, I can play A but I won't write it. Bb sounds way too high, no doubt some pros can do it but I'll set G# as higher in the chart.
Fourth: performing a glissando from the (4th line) D above the break to any upper note (high C - two ledger lines) is the easiest range of the clarinet within which to glissando.
Strange, then was me on my cheap clarinet.
Fifth: Glissandos are much more difficult when descending.
again, strange, I noticed all glisses easier in descendent because was easier to control the pitch by start closing the wholes, than start opening them.
Finally: a glissando on the clarinet should not result solely from "half-holing"... the embouchure and tongue position should do most of the work. I hope this clears up a number of things.
Yes, all this is quite helpful, in order to make a good discussion about this.