MultiDropDown in a child table

We would like have a multi drop down on our child table but that controls is not allowed. Has anyone been able to get this to work? Basically we have a question and answer pair and then would like to select multiple authors from a lookup for each child answer.

Has anyone managed to get any kind of multi-select type data (checkboxes, dropdowns) working in a child editor. The 6.7 workbench doesn’t allow it, but I’m wondering if anyone has managed to get something like this working another way?

Thanks in advance.