If you don’t want to remove it, your only option is to carefully wrap newspaper around each bar, then mask up the tricky bits, ensuring that all is covered, then paint with confidence.
I would make every effort to remove it - I have in the past tried to reach behind using all the above, but there are always bits you cannot get to and some paint will always get on the rad and you will be looking at it for years to come.
If you are going to do it all, do it properly - thats my motto.
That's interesting - I have used them many times and found them to be good enough to reach the areas behind the rads that can be seen - the problem I had was when held vertically the handle would scrape against the wall above the rad - maybe a roller with an adjustable angle would work in my case (just thinking out loud)