In case anyone wants to know what all you can and can’t do so far. You CAN use legendary mogs on characters that do NOT have the legendary weapon themselves.
My Mage has the Legendary staff, but my Warlock does not. My Warlock IS able to use the staff as a transmog!
I think this goes without saying that your character still needs to be able to equip that weapon type. Mages can’t transmog to the Bow or Hammer Ect…
Rogues, when selecting your legendary, right click the picture in the menu and choose the other option. You can only mog the left hand into the left hand and the right hand into the right hand. When you right click the picture, the drop down menu shows up and you have to select the main hand separately.
Demon Hunters are thus far the only ones that can transmog the Warglaives… Because Blizzard are silly like that.
Edit: You do not get any proc visual effects. Shadowmourne will not produce the swirly purple soul effect when using it as a transmog.