Will this isolating valve allow me to change a radiator valve without draining down

In the technical guides for the Giacomini valves, it appears that some of the heads can be removed by unscrewing it past the the open position. Have you tried that? This is assuming that the top part on your model spins.

For other models, it appears the whole head pulls off the valve. An example is shown in this guide: http://www.giacomini.com/export/sites/default/en/download_gallery/certifications/cen_en_215_tech_doc/1257763387429_R452_1-502GB.pdf

In your photo, I can see the tops of the numbers on the valve, but not enough to compare to the various units the manufacturer produces. You might call them for more info at their support line, here:

TECHNICAL MANAGER eng. SERVILIO GIORIA tel.: 0322 923 372 servilio.gioria@giacomini.com

TECHNICAL DEPARTMENT SECRETARY

Mrs. PAOLA FRATTINI
tel.0322 923 372 - fax 0322 923 255
paola.frattini@giacomini.com

I have lived with hot-water heating for 70+ years. I have never seen this valve used. I have been strongly advised to never touch them, since the packing washer will have hardened decades ago, and turning the handle (if possible) could produce a permanent leak out of the system around the stem. All my systems have been one boiler-one circulating pump, feeding all the radiators, and one thermostat to switch the boiler on/off...

If you chose to remove the valve, you will need to shut off any make-up water supply, and drain all the water from that part of the system at or above the level of the radiator valve.

It is quite likely that you will need to loosen the large square fitting to access the washer that shuts off the flow of water through the radiator.