In that example, do you not see using S3 for that purpose as trying to use the wrong tool for the task at hand. Using AWS SSM parameter store [0] seems to me that it would be a tool designed to fit that purpose nicely.
I know it's not the best way, but you could try raising an issue against an active project in the same area using HMRC's github?
e.g. https://github.com/hmrc/auth-client