Eggwash/Milkwash on Partially Baked Quickbread?
Arlo, assuming you're talking about relatively dense quickbreads such as zucchini, pumpkin or banana bread or the like, I have not had any trouble with collapsing.  I use egg or egg white wash sometimes(*) around the time I do my first toothpick test and it comes out just a bit wet.  So, call it maybe 7-8 minutes before it's finally done. I've not tried a milk wash.

Honestly, while this does give it a nice sheen, these breads are so dark already that it doesn't really do anything for the color.

For a sweet bread like banana etc, you can also get a nicer looking top by evenly sprinkling fairly large crystal sugar across the top of the dough (batter) before it goes in.

(*) If I'm presenting to non-family members like at a work potluck.  For the kids, who cares?  Haha.