Will Unity have a keyboard shortcut for accessing the "Session Menu" that appears on the panel?

Solution 1:

In Unity, the Session Menu, and other indicators, are standard menu items. You can reach the menu bar with F10 or other Alt-… combination; for instance:

• Alt+F ← ↑ ↑ Enter

will generally get you to the Shut Down… entry by first opening the File menu and then jogging across via left-to-right then top-to-bottom wrap-around.

Solution 2:

Just worked out that the HUD does what you want. So press Alt and type "sus" (or "shut" ...) and you will get "Device > Suspend" (or "Device > Shutdown ...") as the first option. Press Enter and off you go :)

This bug report (comment #2) pointed out this way.