I haven't had a lot of time to work on this, but yesterday while I was doing something else I put a piece of 1/4" aluminum on it to see what happens. I wasn't expecting it to work, I assumed it wasn't rigid enough to actually mill the aluminum, but it worked surprisingly well.
I used a 1/16" end mill in a 0.3mm depth cut. It didn't plunge perfectly at the start, there is a small ramp down at the start of the cut to the full cut depth, it looks like it got to the full depth after about 10 degrees around the circle. This is probably due to a combination of the zip tie mounting on the Dremel and give in the linear rod/bearing setup. It also doesn't help that the Dremel is still not vertical, it leans forward a little bit, I need to get some shims in behind the Dremel at the bottom and then adjust the cad drawings for the Dremel mount.
I am hoping to get more time over the next week or two to work on this. The next step when I get a couple hours is to try milling new end stop boards. That should let me see if anything goes wrong when I run it through all the steps from milling the traces, to drilling the holes, to milling the outline of the board. I will record that process and post the video when it's done.