Another option is to use a multiAnswer object made up of popUps. Instead of having checkboxes you could put a yes/no dropdown next to each answer, and then use a custom grader to provide the custom feedback. It would also give you more flexibility around the grading, so you could award partial credit if you want.
WeBWorK Problems
Getting answer text from new_checkbox_multiple_choice
This forum has a limit to the number of forum postings you can make in a given time period - this is currently set at 10 posting(s) in 1 day