^ creative solution. I like.
For some reason, when I ordered 0.6mm boards, my fab recommended silver plating over HASL. Probably something to do with cracking of traces?
For thinner boards, I sometimes make them myself IF they don't need a lot of vias or any soldermask/silkscreen. I easily do 8 mil (~0.2mm trace/clearance) on boards all day. On a board this tiny, I imagine I could easily do 6mil traces with high yield, if I didn't mind screening them more carefully.
But... humongous vias, yeah.
One good thing about boards that are really thin, you can easily see your registration on the vias and fudge them a little when drilling. So if thin enough to be transparent, you can make your vias quite a bit smaller.