This feature has been developed and merged into the develop branch of WeBWorK: https://github.com/openwebwork/webwork2/commit/cf5e970a6cfef2a6714e8656191a8b14526baf62
It is not part of Release 2.9, but should be a part of Release candidate 3.0. I also blogged about it with an overview of the details here: http://www.cmhugheswebwork.blogspot.com/
WeBWorK Problems
A "solutions manual"
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