Can you present data on non-participants?
This is standard practice. And it is necessary in order to detect selection bias.

The CONSORT initiative (Consolidated Standards of Reporting Trials) provides a flow chart that includes entries for "declined to participate" and dropouts in various follow-ups.

The American Psychological Association, in its Ethical Principles for Psychologists and Code of Conduct, section 8.05 "Dispensing with Informed Consent for Research", writes:


  Psychologists may dispense with informed consent only (1) where
  research would not reasonably be assumed to create distress or harm
  and involves (a) the study of normal educational practices, curricula,
  or classroom management methods conducted in educational settings; (b)
  only anonymous questionnaires, naturalistic observations or archival
  research for which disclosure of responses would not place
  participants at risk of criminal or civil liability or damage their
  financial standing, employability or reputation, and confidentiality
  is protected; or (c) the study of factors related to job or
  organization effectiveness conducted in organizational settings for
  which there is no risk to participants' employability, and
  confidentiality is protected or (2) where otherwise permitted by law
  or federal or institutional regulations.


The kind of summary statistics you cite falls squarely under point 1b.

Of course, you do need to exercise some caution, in particular to ensure that confidentiality is maintained. Listing the names of non-participants, for instance, would certainly cross the line. As would giving out "too much" information from which one could reasonably deduce the identities of participants or non-participants (if you work with a small, easily identified population, say).