Sorry I don't have ideas as such, but I have paid for engraving that was done on a machine with a floating "cone" type head (like in your photo).
Just be aware that it can cause a lot of scratching, especially in metal like aluminium as the cone will drag chips around.
Engraving a good flat surface is done much better with a normal CNC Z, the benefit of the floating cone Z is for uneven surfaces (which I guess you already know).