Today I put together the prototype. I used the same analog concept as discussed before. And this is how it looks:

I opted to use only two probes, opposed to full 4-wire connection. From first tests it looks good enough - I'm able to discriminate as much as few as milimeters of copper 0,3mm (12mil) wide track on PCB.

Closer look

And here is bottom side of the board:

With detail of MCP73831 in SOIC package

Well, who doesn't feel to do this, can use adafruit LiIon backpak instead.

I wrote first quick and dirty firmware for it and it looks quite usable. After some cleanup, I'll set up github repository for it. But before it, I'm going to heat up 3D printer and design some package for this gadget.