You may not get much from an unplayable rule, but what about the fence itself? Is it considered an obstruction from which you would get relief? If so, there's no restriction on distance for relief:
(i)Through the Green: If the ball lies through the green, the player must lift the ball and drop it, without penalty, within one club-length of and not nearer the hole than the nearest point of relief.
The “nearest point of relief” is the reference point for taking relief without penalty from interference by an immovable obstruction (Rule 24-2), an abnormal ground condition (Rule 25-1) or a wrong putting green (Rule 25-3).
It is the point on the course nearest to where the ball lies:
(i) that is not nearer the hole, and
(ii) where, if the ball were so positioned, no interference by the condition from which relief is sought would exist for the stroke the player would have made from the original position if the condition were not there.
I've seen guys go 40 yards to get relief from grandstands.
Typically you do not get free relief from boundary fences, so you have to rely on the unplayable lie options.