@ akis
I don't think you explained this very clearly.
With a laser level, you have to get the level itself level as well as getting at the right height.
If it's level in two perpendicular horizontal directions let's call them NS, EW, the beam will be on a plane that's level. N is the direction the light comes out of the level.
There are two sorts of cheap level;
The ones with a pendulum, where the pendulum sorts out the NS,EW business and you only have to worry about the height. So if you tilt them, they compensate automatically within limits, and the beam is still on a level plane. They are levels. There may be expensive ones ones with parameters you can dial in, I don't know, but it's worth checking. In fact considering this problem altogether I think you should look into sophisticated laser levelling and sighting gear before trying to build anything, unless you like building things.
The cheaper ones, like I've got, which have two spirit levels, NS, EW, and they come with a camera tripod socket and a little table tripod. You put them on the tripod and fiddle with the ball and socket until the bubbles are in the centre of the lines. They are better used on a proper camera tripod, with height adjustment, which costs a lot more than the level. I'd imagine the dearer versions come with their own purpose designed tripod and something better than spirit levels.
So if I've understood it right, you might be in a large room and due north, 20ft up and inaccessible, there are two marks on a wall a few feet apart, and you want to know if they are level.
Well you could do that in principle with the two spirit level type level. You position yourself between the two marks and tilt EW to get level in that direction and then tilt NS vertically until the beam hits one of the marks. If they are level the beam should hit both marks. You could also design an instrument with a pendulum type level or an electronic inclinometer or something, where you put the beam on both marks and it tells you the angle to the horizontal between them.
OK in a back of a fag packet sort of way, but when you think about the geometry, and the way errors could creep in generally, maybe not so great.
I've never looked into them much, but I wonder if you couldn't do this with an old school theodolite, or the new school laser equivalent anyway.
But I'd have thought that Gyro's solution of a telescopic pole support and camera mount would be better and for only £27.