Can I use the prepopulate module on an entityreference field in an entityform? It may be wishful dreaming, but it would be sweet if I could pull this off.  I've been banging my head against it for a while now.  

Has anyone done this before?
I actually did this on a project recently.

You can use Entity Reference Prepopulate. It's nice because you can make the field disabled if it's populated, all handled through the UI.

The alternative would be to just write a little bit of JS that disables the field on the node edit form if the value isn't empty.