But they mostly still exist, somewhere, right? There must be a way, even if they aren't proper attachments as far as SMF is concerned. How about something like...
1. Build an index from the old DB, to get a filename for each old attachment ID.
2. Store the attachments somewhere accessible.
3. Modify the SMF page display code to replace [ Attachment Invalid Or Does Not Exist ] nnnnn[/attach] with an IMG or URL tag.
... and replace direct links to attachments similarly.