although it may depend on how long since the stroke that isn't sounding too good is it. If you really aren't getting any muscle contraction at all the probability of useful recovery is poor. a number of prognosis studies of the upper limb after stroke, using absent muscle function as a prognostic indicator, have shown this impairment to be highly predictive of failure to obtain functional recovery.

By the way use of NMES may not be an absolute contraindication in relation to pacemakers:

Safety of a combined strength and endurance traini... [Wien Klin Wochenschr. 2003] - PubMed result

Elsevier

...although these studies are small for harm studies and a bad outcome is so serious. I wonder if NMES was applied quite distally (not shoulder stimulation) under cardiology supervision if it might be ok?

If the patient wants you to persist even knowing the likelihood of a poor result how about other alternatives:
EMG biofeedback
Other stimulation techniques like using a muscle vibrator, tapping or ice stroking, combined with Instructions to voluntarily contract.
what about mental practice: this has some evidence for it although personally I find you usually have to have something to start with. NB there is quite a skill to using mental practice - you really have to get the patient to focus hard on the muscle contraction and up the motivation -0 sort of personal trainer approach.