This may not be exactly what you want, but it is possible to give incremental credit and withhold feedback. For example, you can use the custom problem grader fluid in PGgraders.pl (the newest version is necessary, see http://webwork.maa.org/viewvc/system/trunk/pg/macros/PGgraders.pl?view=log). An example is given at
http://webwork.maa.org/wiki/PopUpListsLong
and the relevant portion of code is:
loadMacros( "PGstandard.pl", "PGchoicemacros.pl", "PGgraders.pl", );
install_problem_grader(~~&custom_problem_grader_fluid);
$ENV{'grader_numright'} = [2,5,7,8];
$ENV{'grader_scores'} = [0.1,0.6,0.8,1];
$ENV{'grader_message'} = "You can earn " .
"10% partial credit for 2 - 4 correct answers, " .
"60% partial credit for 5 - 6 correct answers, and " .
"80% partial credit for 7 correct answers.";
$showPartialCorrectAnswers = 0;
ANS( ... );