Effect of pupil responses on the electroretinogram
Answer version1 (read the comments, I leave v1 for the those who can make the same assumptions in the future)

To answer the question we should clarify some general concepts and processes mentioned it the clause:


EMG is performed by means of direct placement of the electrode (the are two types here - surface-EMG and needle-EMG) on/into the skeletal muscles, this is why there is no such thing  "iris-sphincter-related EMG": the sphincter is inside the eye and there is no way to contact it without eyewall penetration somewhere. Thus, this part of the question is hypothetical. 
ERG is performed after pharmacological mydriasis (pupil dilation to ensure retinal mass response or to perform multifocal ERG), so in the standard settings there is no substantial pupils reaction during the test. Thus, this part of the question is hypothetical too.
ERG measures electrical changes when one of the electrodes is directly attached to the cornea - here we should stress "to one cornea", and "reads" the current from only one side. This is why, the reaction of contralateral pupil, skeletal muscles, heart/defibrillator activity are irrelevant since they are out of the electrical loop existing between the electrodes (the second electrode is on the forehead).
ERG measures electrical activity only - there is no muscle contractions in the process, while EMG involves the time needed to acetylcholine to be released and muscle contraction to begin. This means that the time scopes of these tests are different. In addition, as stated, pupils react as an efferent organ (III nerve, mediator related), while retinal photoreceptors react directly to the light stimulation, so there is a clear delay between these two processes. Theoretically, one can perform full cut of the optic nerve with the resultant loss of afferent signal and consequent loss of efferent pupil reaction, but the ERG will still detect the normal curves if the photoreceptors are intact.


Back to question itself: according to the mentioned facts we should state that the question is more hypothetical and theoretical, thus answering it directly is problematic and the answer should not be assessed without facts mentioned above.

So, the final answer - 1+2: No, pupil reaction does not affect the ERG in the real and standard settings and should not be counted as an interfering factor.

Version2

I've found an interesting patent claiming that pupillary responses can contribute to eERG (electrical stimulus instead of light). These influences stated to be artifacts, thus the authors raise the question how to diminish/filter them out.

They say 


  Eye movements and pupil responses may contribute to the eERG; 
  
  Pharmacologic dilation abolished these pupil responses;
  
  Bilateral artifacts, such as pupil responses, in the corneal eERG
  cannot be sufficiently reduced by using dilation drops; Even after
  dilation, a residual, but substantial electrical response persisted in
  the contralateral eye.


So, I should convert my answer to yes/yes. Taking into account the obscure nature of the subject, I hope you will agree that by making mistakes we can learn something new. I personally learnt a lot trying to answer this excellent question.

Patent application number: 20120143080 (if the link will be broken in future, one can find the patent by this number).

Direct link
http://www.faqs.org/patents/app/20120143080