Hi Louis,
In the development branch of the CVS, gateway/quiz assignments in WeBWorK acquire the ability to hide students' grades and work on completed assignments. This isn't completely stable yet, but should be in the next release, which I think is likely sometime late spring.
This isn't quite what you requested, in that it only applies on a per-assignment basis and only for gateway/quiz type assignments, but it's a start and may fit with what you need. If that sounds close but not quite please let us know.
Gavin