Hi Nos, so really instead of trying to set up a script that will modify the UI like that, I'd instead recommend installing the UnwrapCurve plugin from here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=5136.1

Then when you want to know the length of a curve, select it and trigger your shortcut for UnwrapCurve, that will generate a line of that length and if you want to know the numeric length just select the line and for lines the length of the line is displayed in the properties panel size line already.

- Michael