I will leave this post as a reference for those who need their installer postflight scripts (and uninstallers) to manipulate login items in OS X.

Add login item

osascript -e 'tell application "System Events" to make login item at end with properties {path:"/path/to/itemname.app", hidden:true, name:"Customized Name"}'

The name property can be omitted.

The hidden property is useful for applications that do not have LSUIElement set, it causes them to start with all windows hidden (what Cmd+H shortcut does). Please be good to users and set it to true if you are adding non-LSUIElement app into login items.

NOTE: If path is symlink, OS X will resolve it, and you will not be able to delete the item later by that path. You can use name property to locate the item.

Delete login item

By path:

osascript -e 'tell application "System Events" to delete every login item whose path is "/path/to/itemname.app"'

By name:

osascript -e 'tell application "System Events" to delete every login item whose name is "Customized Name"'

The every login item may be changed to first login item or last login item if you are afraid of deleting too much.

Get list of all login items

osascript -e 'tell application "System Events" to get every login item'

Add whose property is ... clause to narrow down the search.