Make the node type form fieldset more customizable

I'm currently working on some extension to Organic Groups and I planned to add some settings to the group settings on the node type settings form but realized that my module executes later than Organic Groups.

I know that I can adjust my module to execute after Organic Groups - but a more beautiful solution would be if og_form_node_type_form_alter() respected existing content in $form['og'].

I haven't looked at the other form-modifications in OG so perhaps this is true for other functions as well - anyhow I'm attaching a patch for this function.