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Blacklisted phrase (0.5): thanks
Long answer (-0.5):
Has code block (-0.5):
User mentioned (1): @rioV8
User mentioned (0): @starball
Self-answer (0.5):

I now have 2 working solutions, thanks to @rioV8 and @starball:

package.json - add the following contribution to the correct section:

"configurationDefaults": {
            "[ABC]": {
                "editor.minimap.markSectionHeaderRegex": " ... regex goes here ... "
            }
        }

activate() - Additional code within the extension:

const scope: vscode.ConfigurationScope = { languageId: "ABC" };
const inspect = vscode.workspace.getConfiguration("editor", scope).inspect("minimap.markSectionHeaderRegex");
if ( !inspect?.workspaceLanguageValue )
{
    vscode.workspace.getConfiguration("editor", scope).update("minimap.markSectionHeaderRegex", " ... regex goes here ... ", vscode.ConfigurationTarget.Workspace, true);
}

I went for solution 1. as it seems to be the cleaner solution for the extension I'm working on.

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