Dear Peter --
Isn't McDonald's a possessive already? So, McDonald is the nominative, and McDonald's is the genitive. You cannot say McDonald's', can you!?
You see -- I am not a native speaker, and I sometimes realise how particular declensed words can be perceived as if they were in the nominative case, just for their common wide-spread use. I mean, people never see McDonald written without the <'s>, and may thus forget that <'s> implies an original understood "place". As it happens for "the newsagent's".
So, my question is -- do you (native speakers) see the word McDonald's as a whole, or perceive what the 's means?
Forgive the stupid question -- but sometimes English can be really puzzling.