Does persistent focal slowing without epileptiform discharges indicate increased seizure risk in patients without significant structural abnormalities?  

Good question! :-) Lateralized rhythmic delta (LRDA), especially temporal (TRDA) certainly does, basically to the same degree as interictal epileptiform discharges (IEDs). Focal polymorphic slowing is of course not technically "epileptiform" but it does indicate some focal dysfunction, so I would ...

I would say no. Persistent slowing usually suggests a structural lesion. Slowing can denote an increased risk of epilepsy only when it is intermittent and rhythmic, like temporal intermittent rhythmic delta activity (TIRDA).