hook_form_alter() "og_group_ref" editable field's #options?
It's not a form alter but it's an easy solution using rules.. I added a second entity reference field that only listed the nodes I wanted. Then I used two rules to make sure the two fields always match, whichever is updated.

This is one of the rules for when og_group_ref changes. The other one is the same except for when the second entity reference field changes.



Why am I posting this as a solution when it's different to the question? Because it's very quick, most people use rules and it's applicable in many situations.