If anyone else has this problem, the solution is adding a line to:
…/public_html/modules/Contacts/vardefs.php
‘opportunity_role’ =>
array(
‘name’ => ‘opportunity_role’,
‘type’ => ‘enum’,
‘source’ => ‘non-db’,
‘vname’ => ‘LBL_OPPORTUNITY_ROLE’,
‘options’ => ‘opportunity_relationship_type_dom’,
‘studio’ => true,
),
I added the last line studio=>true and it’s now available in Studio to add to the layout.