How to customize "Build number format"

So far, unless I'm missing something, I can have 2 build number formats:
1. Something like this: {0}
2. Something like that: {build.vcs.number.1}

But we have our own custom build number / label.

Here is what we do:
1. Before the build starts we generate our own custom build label and label all the code.
2. Build.
3. Would like TeamCity to display out custom build number instead of a counter or VSC changeset.

We could store custom build number some place (i.e. Computer environment variable [not to be confused with TeamCity environment variables which looks like rather something else], a database, text file, etc).
But how to get TeamCity to display that custom build number?  Is there a way?

Thanks much.