I don't think the HyperSpec allows for that interpretation. The entry for slot-value is specific that slot-unbound is called when a slot is unbound, and the entry for slot-unbound specifies that the default method signals an error of type unbound-slot. There are no exceptions specified for unsafe code. This is actually important, since the whole slot-unbound and slot-missing protocols can be used to define reasonable semantics, for example to define hash-table-based slots, and user code needs to be able to rely on the correct workings of user-defined extensions here even for unsafe code.
However, the entry for defclass allows for adding new class options or slot options, with which you could enable such an optimization (which then becomes a safe assumption for user code).