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).

It seems to me that this is clearly relevant to the study. As long as no personally identifiable data (and just the numbers cited clearly don't qualify as that) I don't see a problem.

In the end, "declined to participate" is an indicator of some self-selection, which could introduce some bias.