[support] Changing checkboxes options during hook_node_validate

I have a node form that contains a checkboxes field with options NULL at the time the form is displayed.

During node_validate, if certain criteria is met, an array of options (checkboxes) is created, and the form needs to be redisplayed with the checkboxes as part of it.

I've tried setting #options = the new array in the $form, in $form_state[original_form], etc all to no avail. The field doesn't change.

I also set one checkbox in the field at the start, so that I could find it in the $form and $form_state structures, changed where I found it to the new options array, and again, no change. I am setting $form_state[rebuild] to true, and even tried explicitly calling for the form to be rebuilt.