I've simplified it more:
There's definitely something weird going on here...
If you open the console, you'll see there's also an infinite loop happening when I directly embed the templated html.
But I'm able to {{ object.property }} in this case, even though the object is null in the filter.
I've narrowed it down to something in polymer vs. platform (not very narrow).
I'm trying to dink around with it a little...I've got a mini test environment set up where I load all the individual source files instead of polymer.js.
But thats still quite a few files (everything in polymerDev/src (declaration, instance, lib) and polymer-expressions.js) that could be the culprit, so I'm not sure how far I'll get.