Migration Assistant from Carbon Copy backup? My Mac has died and I'm considering whether to fix and restore to the same hardware or get a new one and import data.

The TL;DR: Do you know if Migration Assistant will take a bootable external disk with the data as source? 

The long story:

I have a fairly recent Carbon Copy backup of my data (on El Capitan), but if I get a new Mac it'll be running Sierra, so restoring the HD would be a no-no. Also, it would be a good moment to get a "clean" OS baseline to work from, as long as I don't have to go back and reinstall everything. 

I know that Migration Assistant can take a disk image file, but I'm not so sure if it'll be able to read a external HD as source?
You can select a drive in Migration Assistant.  That is how I transferred my old system to my new one.  I simply mounted the drive of my old system on my new system, and then was able to select it as my migration source.