Is there any way (plug-in etc.) to get DsE to provide a preview of Outlook emails, short of converting all the email to Apple Mail or some other format?
You mean while they’re in Outlook, ie. not imported into DEVONthink? If so, DEVONthink doesn’t do that for Apple Mail either.
The only possibility would be to install a third-party plugin for Outlook emails (.msg) but I don’t know if there’s one out there.