Ok so few things
Did your custom attributes have been added to the slax metadata(cmdb_metadata_slax.htmpl)?
To address your custom attribute you need to go the extended table using dotted notation. For the slax table will be assoc_slax.z_productname
You can't use parameter in you macro that don't exist so the list_display="z_productname", list_orderby="z_productname",rel_attr_name="id",use_list_display="1" can't be used there
Not in front of a dev machine to test but the below will normally do it assuming you are also filtering on your z_productname(otherwise if you filter on CI name your previous whereclause must be ok already):
<PDM_MACRO name=dtlDropdown hdr="Test" attr=z_IT_service.assoc_slax.z_productname factory=slax lookup=no make_required=yes link=yes whereclause="assoc_slax.z_productname='SERV-IT-000001' or assoc_slax.z_productname='SERV-IT-000001'">
Another think to optimize(but need an extra field) will be to create an integer flag on the nr object where you can flag the ci that need to be display or not this way you will only need to have a simple where clause likeand you don't need any other relation and attributes in th extended table:
<PDM_MACRO name=dtlDropdown hdr="Test" attr=z_IT_service.assoc_slax.z_productname lookup=no make_required=yes link=yes whereclause="z_display_flag=">
Another way again to do this may the above not work will be to construct your own list with PDM_LIST and use js to construct your dropdown. giving you more control on the what you want to display from foreign table. (will be for another post if still needed )
Hope this help.