Not being able to move Mjolnir doesn't have anything to do with it's weight. It's not like it suddenly becomes a million tons or anything (otherwise it would continually create craters everytime Thor dropped it). It's simply Odin's enchantment that stops anyone from moving it who isn't worthy.
I'm pretty sure that after Thor hung Mjolnir in the coat hanger no one would be able to remove it from there if they weren't worthy.
I loved that Ultimate Avengers movie... up until that stupid scene. I kinda felt like they did that just to get Thor out of the picture so that Hulk could continue dominating the other Avengers.
To be fair, in almost any animated production, whether it be animated movie or TV series, the director/producers always seem to crap on Thor whenever Thor and Hulk fight.