Review-only supplemental material
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?