Review-only supplemental material I am at a difficult position. I work in a major software/telecommunications company, and pursue a doctoral degree at the same time. The company has strict policies against competing behavior. I have implemented a lot of core Internet component code at my free time, but the company would consider releasing those under an open source license as competing behavior. I won't consider changing the employer, as the salary is acceptable, the job is extremely interesting and I frequently get good extra payments for all of the inventions I have made.

However, I have managed to obtain publication permission for several articles given that main algorithms are only explained as pseudocode. Because my core Internet component is over 20 000 lines of code, I of course cannot explain all as pseudocode.

I would like to submit articles related to this to major IEEE computer society and communications society journals. I would like the reviewers have ability to assess that all experiments have been performed correctly, and thus see the source code for the experiments made in the articles. But I cannot according to the company policy publish the source code to all readers.

Now, what I would like to have is review-only supplemental material. Supplemental material that only the reviewers can assess, and that the journal can store for their private use (e.g. for verification of results if there's a suspicion of scientific misconduct), but not available publicly for all readers.

Is this kind of review-only supplemental material in general possible? I wouldn't be surprised to find if the answer was "no", as it makes it impossible for regular readers to work as unsolicited reviewers, publishing their own commentary on the results in the process.

Of course the answer can depend on the circumstances and the journal, so perhaps asking the editor would be a good option. But I believe this question may have more general value, and thus, I am asking it here as well.
Technically, there is obviously no problem. In the paper, you provide a link to your server that will be accessible only by reviewers. After review, you block the link. You may explain in the paper, that the code is proprietary and will not be accessible to readers. As soon as this is not a mandatory policy of the journal to provide an open-access code,  I do not think that you violates any journal policy. You will violate for sure the policy if you submit supplementary material, and then refuse to publish it. I suggest that you write to the journal that you plan to submit and suggest such a strategy. But do not explain the whole story - no one will read all these details. Just tell them that you have proprietary code that you can share with reviewers, but cannot make it open to all. Is it OK?