Seek and ye shall find! I was just able to get it to work using my converted Snoop project (which targets .NET 4.x).
I had a null reference exception that was hiding the fact that it was working yesterday with this same approach, but with a fresh mind, I was able to see that null reference exception (related to a focus feature I'm integrating in) and kill it.
So, it is verified then ... the current version of Snoop that is out there ... will not work against WPF 4 applications.
I'll let everyone know when I get everything up there ... but it makes a lot of sense now to also publish (on CodePlex) a version that works with WPF 4. I'll add that to the list of things to do on this matter. Hmmm. That list keeps getting longer. :)