This Instructable may damage your espresso machine. Probably, most of baristas will not add anything than coffee in the filter basket: if you put fine granulated ingredients that can pass trough the head mesh, and have a reflux into your machine's head, it may harm your head, pipes or even boiler (I will dose the ginger between two layers of coffee so I will minimize the chances of wreking up my machine). This is not likely to happen, but you must consider this. I'm using an old Saeco machine, soon to be replaced.
The other solution is that if you like your coffee with milk or cream you could easily add the ginger essence to that. There are some people who are coffee 'purists' who don't like to add milk/cream but then THEY probably wouldn't add ginger. Or of course if you like your coffee black then only using your technique or using an aeropress would work.
MUST get round to experimenting. Will let you know the results.