mochitest-plain and e10s OOP tabs

I've been poking around at getting mochitest to run in e10s with OOP 
tabs enabled.  There's quite a lot of tests that don't work right now, 
since various features aren't available.  But one of the obvious 
problems that pops up is that many of the mochitest-plain tests use 
UniversalXPConnect (mostly for managing preferences and initiating 
events using EventUtils.js), and these tests all fail with the message 
"A script from http://localhost:8888 was denied UniveralXPConnect 
privileges."

Can someone confirm that this will remain the case, and that content 
will not be able use XPConnect in an e10s world?

If so, I propose to deal with this by moving the affected tests (~370 of 
them) to mochitest-chrome.  There are possibly other ways to deal with 
this, but this seems to be the most efficient fix.

Jonathan