As of today’s (16/1/19) update buses L3 and UL16 have been added to the feed, being rail replacements for Chingford/Liverpool Street and Romford/Stratford, respectively. There are also appropriate entries in the /Line/Timetable feed.
Is this a first step in adding all rail replacements to the feed? If so, will they appear in (I guess) the Temporary Bus Change notices at the appropriate dates?
In the absence of anything else, there doesn’t seem to be an easy way to distinguish them from real bus services.
I note that, as a consequence, they are also in the bus map section of the Tfl website and appear to be regular services.