I've read this argument made for the mega fauna as well, in that stuck in my mind is that avocados seeds were spread in the dung of mammoths..
The avocado stood out in my memory, too. I'll have to try to devote some time to finding what called this to my attention.
In the meantime, are there are clear arguments against this proposal? Something that might be used by a creationist or similar denier of reality?
Good question, mega fauna haven't been gone long in geologic time, and the fossil record of them is quite large to say the least. Some "renegade" scientists want to either resurrect the mega fauna and turn them loose in wild areas of North America or transplant things like elephants and such. I have mixed feelings about that...
There are clearly plants that no longer have animals to disperse their seeds, osage orange is another that comes to mind. I'll have to google a little and see what I can come up with...