Nested polymer template inheritance
This used to be possible in Chrome Canary (see this blog post, this discussion, the fixed spec bug, and the fixed Chromium bug), but unfortunately it doesn't seem to be working for me anymore in Chrome 33.0.1750.22 dev on Linux or Chrome 34.0.1809.0 canary on Windows with the "Enable experimental Web Platform features" flag on or off. Try this jsbin to see if it works for you.
Update: Confirmed. This feature was removed from the initial implementation of Shadow DOM in Chrome due to "implementation concerns."
I guess you would need functionality like the select='selector' attribute of the <content> tag for the <shadow> tag where you can select a few nodes from the older shadow and position them with one <shadow> tag and select and position the rest with a second <shadow> tag.
I haven't found anything about this though and I am quite sure that this isn't supported.
This discussion might be interesting https://groups.google.com/a/dartlang.org/forum/#!topic/web-ui/zR64jebST4c