I would like to change the position of my seat-post lever but in order to do so, I have to untighten its screw.
The problem is that I can't manage to access the screw, due to the presence of the hard metal cable going to the lever: it's right over the screw!
Here is a picture:
So here is my question, what's the type and size of the screw? How to access it?
In my line of work as well as on bikes, a cut down Allen key can be very useful. A dremel or good hacksaw applied to the short end of the tool can be used to make it even shorter, just be sure to tidy up the cut.
In your case the long end of the Allen key could then stick out at about 10 o'clock (in your picture). You'd only get around 1/6 of a turn at a time but that's OK.
Another useful tool (though not always a good idea on tight screws) is a ball end Allen key. A set is well worth having. These allow you to drive the screw without the tool being among the screw axis, so once the screw is slightly loosened with the cut down key, you could easily take it out with a ball end key.
Have you tried to use a hex key with a ball end (see image below)? You may insert it askew into the bolt's socket and turn it. The ball shape works like in a Cardano joint (aka universal) when you drive the bolt with it.
It looks on your photo that there is enough room right at the bolt to insert a tool. While there is not enough space in a straight line from it.
@Chris H mentioned in his answer that using such a tool might not be a good idea for a tight screw. The reason is that compared to a regular hex driver the ball engages with a much smaller area in the hex socket of the bolt. One may mangle the socket when applying more than a few Newton-metres of torque. Soft materials in screws and tools that are not well made exacerbate this.
It might be a good idea to apply a few drops of a penetrating oil with a solid lubricant, for example MoS2. Put it at the base of the screw such that it gets pulled by capillary action into the threads.
