If you select your “control number” file in your Query with the *ALL option as matching criteria, then your control number will be available on ALL records of your report. So you should be able to use it into your header.
I don’t know of a way to get a variable value into headings of a Query/400 query. However, if you convert it to a QM query, you can use the &col text variable syntax to place that column value in any heading line you create. The value could be obtained by joining to your one-record file or simply by passing it in as a parameter to the query and referencing the substitution variable in the SELECT that’s created. I think it needs to be referenced in the reporting in detail lines, but detail printing it can be suppressed simply by assigning a zero width.