How to set an individual IGUIElement its transparancy?

[realmsinthemists]

And not all of the others which happens when you use the skin.

[CuteAlien]

Some elements have setOverrideColor functions, thought not all (more in svn trunk version of Irrlicht).
Otherwise it's tricky. You could override the skin and check for your specific element (and call default-functions of default-skin otherwise).
Or you could copy the code of a gui-element and modify it in any way and create your own.

[realmsinthemists]

(more in svn trunk version of Irrlicht).

Does this mean it will be in the next (or the next after) release?

[CuteAlien]

Yes, svn (https://sourceforge.net/p/irrlicht/code/HEAD/tree/) is what Irrlicht 1.9 will be. But you can also just use it now (I work with svn version all the time in my projects).