Literature on English pro-drop

I’m trying to track down the literature on analyzing verbs like “sing” that can occur with or without an object as object-dropping verbs. For example, the ERG provides only one analysis for “Kim sang” and “Kim thought”, where the transitive verb drops its object. Can anyone point me to a reference that argues for treating this as object dropping rather than homophony (ie an intransitive verb and a transitive verb in the lexicon)?

The place that I would start is the Fillmore paper on null instantiation:

There are probably more up to date discussions in the FrameNet literature, such as: