After digging a bit through the source code I see the "properties" search parameters gets handled in row 1584 and downwards in include/search_functions.php where the cu part gets split up and put into a case switch resulting in this trigger
case "cu":
$propertiessql->sql .= $sql_filter_properties_and . " r.created_by = ?";
array_push($propertiessql->parameters, "i", $propertyval);
break;
So it looks like my "best" workaround would be to add a case like "ncu" or something similar that runs the almost exact same query but uses r.created_by <> ? instead.
I suspect that is the best solution as I doubt this is something many else wants so I'm not going to suggest adding it to the codebase.