Just like its legal to lay your cue on the table to sight a bank shot angle, as long as you don’t let go of it, the same should likely apply to using a ‘ball-in-hand’ cue ball to measure with. But if you do, you probably shouldn’t be allowed to let go of it unless you shoot it from that location. The same would also apply to using the triangle to determine a break ball (if you are going to be at all allowed to misuse equipment to measure). At least hanging on to it gives you a pass to violate the spirit of the rule.In my view any way you want to use your cue stick and arms/hands/eyes to plan and align shots should be perfectly OK.
A fairly common kicking system requires you to shoot towards a point that is the mirror image of the object ball reflected through the cushion. There are lots of YouTube examples of players measuring out the right distance to go on the other side of the cushion with either their hand or the cue stick. I have never seen a foul called for such a thing.
I also think that if you have the cue ball in hand you should be able to place it wherever you please for any purpose you have.