I want to know whether or not it is useful to a malware researcher or AV product to use steganalysis tools to detect or study malware. If so, is there a standard steganalysis tool that's generally used or does it vary from analyst to analyst?
Generally malware analysis and steganalysis are orthoginal to each other.
Malware analysis deals with malicious code.
Steganalysis deals with hidden messages.
Yes the hidden message could be malware but it really doesn't matter, it's just a hidden message until it's extracted. For it to run automatically as malware, some other code (malware) would have to do the extraction.
Generally the two don't cross paths but in the rare case that they do, you have malware first to point to the Steg and the malware defines the extraction. No need for a Steg tool.