Place custom content field only on sidebar
You can do this with the Panels module.  From the Page Manager configuration screen (Pages in the admin interface), enable the node edit page override.  Then edit this override.

Create a new variant.  Set the selection rule to be "content has type" -> "my custom content type".  Then, under Layout, choose an appropriate layout (one with a right sidebar).

Afterwards, under content, you will need to add all of the fields for the content type to be edited.  Just place My_field in the right sidebar and it will be displayed there.