Homebrew PCBs

Yes, thicker brass takes a *lot* longer, you want to make sure your
etchant is fresh and hot or it'll be in the solution for an hour or
so.

Undercut is going to be about as much as the thickness of the brass; 8
mil brass means 8 mil undercut if you only mask one side - masking
both sides is better but you need to be accurate within the thickness
of the brass - a few mil in your case.

http://www.delorie.com/pcb/brass/

These days, it might be useful to look for people who have a small
laser CNC and are looking for small jobs to fill in the dead times.
I've seen a few pop up recently - try www.laserkrafts.com or
www.ohararp.com