Using an almost fresh sample database I can place holds on itemtype "Reference" items without _any warning_ on item and record level in the staff interface AND the OPAC. Sample data example: Programming the Perl DBI This should not work as the item type "Reference" is marked as not for loan in administration. These items are also not picked up by the build_holds_queue.pl for this reason.
I mistook the collection Reference for the itemtype... just ignore me.