I’ve checked all the envvars and also all of the build dir (it contains all the stuff autogenerated for the build), and the custom message does not appear anywhere.
Neither does any kind of message appear in build details in the API.
So it’s not available as of now AFAICS.
If you need to customize what the custom build does, you can pass a custom config for that (keep the merge mode in mind).
Related, possible duplicate of: