I can’t see how you’d do it before new mail arrives, but if you are looking at AFTER new mail arrives, you should be able to export the entire document as DXL to a NotesDOMParser. You will be looking for a <filedata> node. The filedata in the node will be Base64-encoded. You can create a temporary in-memory document with a NotesMIMEEntity for a body to decode the filedata to a normal LS string.
Yes, that’s a bit of working around, but if you need to avoid the file system, it’s the only way to go.