Interested in Language
and you will form a liking to TG very soon. Andrew is a cool guy. I like his student-friendly style of writing.
Regarding your question, IMO "was burned" is a quasi copula where "quasi" means the linking-verb retains its dynamic sense -- part of it.
There is a gradience running through from action verbs through to the class of copulas. The distinction between them is not clear-cut.
I don't know when I will have time to study TG, but, in the meantime, wouldn't you agree that Reed-Kellogg has the sizable advantage of not having to include any words in a diagram besides the words of the sentence that is being diagrammed?