Well, in the end I was right.

I closed both valves in most of the large radiators and let a few small ones in different floors drain.

I selected a point to cut the pipe slightly higher than the drain cock.

Sure, there was water coming out of the drain cock in spurs, at random intervals, but it had no pressure.

The pipe was dry at the point of the cut, which allowed me to solder without a hitch.

When I reopened the radiator valves the system had still about 1 bar of pressure, without adding any new water. Needless to say that this saved a lot of time and inhibitor.

Pipe freezing , or using push-fit valve fittings, is something that I am well aware of (I have a pipe freezing kit) but I did want to use capillary because the tees are going to be put under a lot of stress and it give me more piece of mind. Being a 22mm pipe, and wanting to solder, made pipe freezing out of the question.