An oft-requested feature. Attached are two diffs to the 1.9.2 source which enable this. Unfortunately I had to alter weblib a little to make all of the features work, but it shouldn't break any existing code. The other changes are relatively minor. Please note that this patch does not do any validation on the text field inputs, and will silently accept erroneous values.

In order to use this modification, simply patch the files in mod/assignment with the assignment.diff file, and lib/weblib with the weblib.diff file. To allow decimal grades, you must also change the database field 'grade' in 'assigment_submissions'. On MySQL I used
ALTER TABLE `mdl_assignment_submissions` CHANGE `grade` `grade` DECIMAL( 10, 2 ) NOT NULL DEFAULT '0.00'
– your command will be similar.