Please find a very raw concept of such indicator tool here. I'll continue work on this, but your feedback can help me to make this tool more powerful.
When the indicator has just been applied it shows chosen fib. levels for 50 last bars. After that you can click on the chart to choose the bar to find the high/low from and the high/low bar to. To choose you must click the right mouse button over the bar and then choose "Search High/Low From" or "Search High/Low To" commands in the context menu.
The line has two styles. One (by default - solid) is used to draw the line over the chosen period. Another (by default - dotted) is used to extent the lines in future (to the end of the history + 100 bars), so you can easily see which period is chosen.
Lack of the current version of the indicator (things to do in future):
a) If you has applied the indicator twice - two sets of the commands will be show and you cannot recognize which of instance of the indicator will be changed by the command.
b) Every time when you change the indicator (even if you just change a color or switch the instrument) - the indicator is reset of it's initial state, so you have to set the period again.
BTW, for the developers it's a good example of the indicator which can interact with the users.