adding fillet to assembly? how?

Here is a fun one. How would you add a fillet to an edge created from an
assembly cut, in an assembly. In other words, the parts are machined
after being assembled, with a ball end mill, and I need to show the
finished shape with the fillet.

btw, you can't add a derived part back to the same assembly it was cut
from, go figure.

ca

 

Don't do an assembly cut, do a cut on one part, then an in-context cut
on the other.

Unless, of course, something prevents you from doing this.