Manage login items from terminal in 10.11 El Capitan
Well, it seems there's no cleaner way than Applescript - at least my searches have turned up nothing.

So I've written a simple script to add/remove/list login items:

% loginitems -a Caffeine -p "/Applications/Caffeine.app"
% loginitems -l
Caffeine
% loginitems -d Caffeine
% loginitems -l
%