With SLComposeViewController it's possible to post to Facebook without needing a Facebook app ID, and this shows up as "via iOS" on the users wall. Is it possible to do the same with an SLRequest , I.e post with the "default" app, or is an app ID always required for SLRequests?
You have to have APP ID for SLRequests.
For SLRequest, you have to make approval from user and as approval is there, there has to be identity for which approval is being done.
Hence you have to have APP ID to make approval from user for that APP ID.
Regarding SLComposeViewController, it is you have already account in setting for Facebook and you are posting in facebook using that account.
The big difference between SLComposeViewController & SLRequest is as below.
SLRequest : You can post on user account using your own account (using APP ID you created).
SLComposeViewController : You can post on user account using user account.