See T39470 and D436. Code by @tippisum, with some minor edits by @mont29. Tested with various rigs, including Rigify, CGcookie flex rig, and gooseberry/pataz caterpillar. Riggers, please test it, no change expected in behaviour. Reviewers: aligorith CC: tippisum Differential Revision: https://developer.blender.org/D436