Once again I'm beggin' the devs, maybe there is a kind person who could lean on that small part of code, please!
: Not as good as a good hot key toggle, but I finally found a way to bind the mouse steer to the keyboard.
1) Open the \My Documents\The Lord of the Rings Online\lotro.keymap file with a text editor
2) In the section below :
Add the following line : CameraInstantMouseSteer [ User [ 0 DIK_Q ] ]
It will set the Q key as mouse steer, you still need to hold it down though. However it lowers the stress on the hand than having to use the mouse button.
You can change the Q to anything else replacing Q by the letter/key of your choice, be sure the chosen key isn't bound to any other functionality.