1. A reduced scoring period (e.g. late homework counts, but at a discounted rate) see http://webwork.maa.org/wiki/Reduced_Scoring
2. Successively reducing the score (grade) as the number of attempts increases can also be done by writing a PGgrader (like the examples in PGgrader.pl). See http://webwork.maa.org/moodle/mod/forum/discuss.php?d=2691 for a preliminary discussion on this. This will require a little bit of knowledge of how WeBWorK can be extended.
By now either Dick Lane or Paul Pearson may have a grader that you could plug in and use to reduce the maximum grade as a function of how many incorrect attempts the student has already made.
I suspect that the reason it is not easy to find a well documented example on the wiki is that most instructors quickly come to your conclusion that this kind of reduction of points is frustrating and not particularly educational for most kinds of questions.
The discussion above suggests implementing it for multiple choice questions where it would be a gentler version of limiting the number of attempts.
Hope this helps.