Can I create a custom System Workflow Message for a particular Contribution Page?
As I learned from @Nicholai in a previous (unrelated) thread, you can override these templates with some pretty easy logic to add something custom to a single page. For example:

{if $contributionPageID == 20}
{* whatever you need to do on this page *}

{/if}

I assume this would work in this case, too. If you try it and it works, let us know!