Though I am not entirely certain, I am under the assumption that you are wanting to use the Rich Text Control for display purposes only and not for interactive purposes. 

If that is the case, then I would indeed recommend using a Calculated Control instead, but using the custom class to style the control to your specifications in css. 

Doing this would also make it so that you don't have to use a Form Variable to 'transfer' the value from the Choice Control to the Calculated Control, and because you would be using a Calculated Control, your formula could be more robust allowing you to do things like set up a 'default' messaged when no choices have been made. 

The rich text displayed here (MIKE NIELSEN has DENIED this invoice);

Was configured in the control like this:

Is there no way to do this using a form variable?  I've tried connecting the choice control (which sets the form variable) to a workflow variable or a Sharepoint list column (item property) to no avail.  I assume it's because this data isn't set anywhere else in the workflow.  I suppose I can use CSS to customize the calculated field's appearance but I would much rather use the method detailed in the snips above.