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I think the answer depends on whether you mean: would economists and  
others then consider him a marginalist, or would we now, in  
retrospect, so consider him?  
 
I would think that then he would not be. His study of motivation  
would have been considered psychology. The study of psychology was a  
rival to the study of (marginal) economic theory. So his ideas would  
have been viewed as representing an alternative to economics. (Or  
maybe even the study of something wholly separate from economics.)   
 
Today, with the emphasis of "mainstream" economics on questions of  
information and incentives, he certainly could be viewed as an  
empirical precursor of the principal-agent  approach.  
 
So I'd say that it depends on how you frame the question.  
 
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