My solution to this problem, though inelegant, was to fill #__uddeim_emn with entries that corresponded with existing users and potential future users. I dumped #__users to get the existing user IDs, added about 4000 more consecutive user IDs, added a column and set the "status" to "2" (notify when offline), and imported the resulting csv into #__uddeim_emn. As new people register, they will already have a user ID in the UddeIM table.
I figure it will buy enough time for someone with a better head for code than me will come up with the approriate solution. The downside of this is it fills up server space that doesn't have to be used. For me, it beats the alternative of manually updating every new user...
If anyone wants it, I can post the script I used to import the csv file into the table. I orginally found it somewhere on mambers or mamboserver, I think. I used excel to manipulate teh csv.
I'm welcome to better suggestions, of course!