ive done it a couple of times.
if done correctly it does more than hold water in the f and e, it creates a vacuum which allows you to work on the system without a drain down.
it is important to only have one open end at a time though otherwise all you will achieve is keeping the f and e full.
so for a valve swap you would close the valves on both sides of the rad, then when you uncouple the valve which needs swapping from the rad you will just get a few dribbles from the rad which can be stopped with another bung or apprentices finger etc as its not under pressure.
then open the valve that is to be changed and is now uncoupled from the rad and allow water to fill a container until the vacuum from the f and e holds. whip the new valve on and jobs a good un