The answer is yes, it will produce full height QuickTimes that can roll into a grade elsewhere, and presumably the DPX and OpenEXR outputs are fine although I haven't looked at them specifically.
The issue is now to find a way within the innards of FCP X to derive an edl from something... I can't bear the though of manually adding cuts on a long form project in a grade. Of course there are "optical" tools for this - I once used CatDV for the purpose, when FinalTouch was very broken - but it's a bit of a crazy workaround.
I know half of you will never believe this but I had never opened iMovie (why should I?) but a colleague tells me it all feels deeply familiar. Which means if we crack this, we can work with a decade of home videos too! I'm so excited! This is incredible!