My question (even after revision) wasn't too clear. Right now, my shared document uses this for any company-specific text, pulling in separate files:

{IF {DOCVARIABLE CompanyName }="Acme" "{ INCLUDETEXT "Acme numbers.docx" }" "{ INCLUDETEXT "Apex numbers.docx" }" }

The above approach works but of course requires multiple additional files. It would be much better to have the specific pieces of text as part of the main document, but appear only when the appropriate company is set.