I figured it out. I'm writing the lightDM configuration when configuring autologin anyway, and that's where I'm specifying the user, so the right thing is to specify the system default at the same time: wiki.ubuntu.com/LightDM#Changing_the_Default_Session
However, when this bug is fixed: https://bugs.launchpad.net/lightdm/+bug/1371710
I'll need a better way to set a per-user default since lightdm won't be reconfigured/restarted for each user autologin.
More data: with lightDM, a desktop preference will be looked up in
/var/lib/AccountsService/users/$USER (no good docs, but some tantalizing details here), and if not found will be looked up in
$HOME/.dmrc (described here). When a user logs in and chooses a DE, both of these locations are populated. So, a script could provision either of these locations (either using the DBus interface or writing to the AccountService file directly) to set a suitable default for the user.