BuddyPress.org

The html of the custom xprofile fields is generated by yet another function; get_edit_html() in bp-xprofile-classes.php. So if you want to change anything in the custom fields, including the * for required fields, you’ll need to copy/rename that function to bp-custom.php as well.

EDIT: …which is not straightforward:

Fatal error: Call to undefined method BP_XProfile_Field::custom_get_edit_html() in /serverpath/wp-content/mu-plugins/bp-custom.php on line 256