CUSA offers locking mode dial for 5D Mk. II
 

$100 upgrade available for the 5D Mk. II, replaces
its mode dial with a locking one similar to the 60D.

See details at:

Canon U.S.A. : Consumer & Home Office : EOS 7D

(Link URL says 7D but the page applies equally to the 5D Mk. II)

Worth every penny... (though it should have been standard from the beginning)

Probably not a bad idea, though I must admit that, after some 8000 pix in difficult conditions (concerts, so in the dark, hot, and in a hurry), I've never had the dial move inadvertently. Must have been lucky...

Sadly there is no indication that this modification is to be part of standard manufacture of all 5D MkIIs in the future..

Who does install?
 

It is not clear to me in the link if this a DIY install or does the body need to go to a service center for this modification?

It's not a DIY. You send the camera in for the retro-fit. It would also be a
good opportunity to have your annual clean & check done at the same time.