Never mind, guys, I found it – it was indeed a rogue record in dmNavigation_aObjects. I was having trouble tracking it down, so I just emptied that whole table, and then the site root loaded fine. Without content, of course, but I just started reimporting the rows from my backup table, a few at a time, until I finally found the one with the problem. I zapped it, and all is well.
Thanks for pointing me in the right direction, Blair and Jeff! I really appreciate the assist!