e.g. The lecturer gave an encore presentation.
Its usage is certainly more common in the entertainment sector but in that example the content and intent is clear; this is independent of whether a lecture is considered a performance.
To "deliver an encore" as meaning "more of the same" could be used elsewhere, often in the negative form to describe a repetition not occurring e.g. an encore not taking place meaning a sports team not winning back-to-back games.
(That actually surprised me, because I thought that it could probably be used in most senses for again - its literal translation from the French.)
Note also that the cited dictionaries also all refer to the use of encore as noun, verb or exclamation - there is no suggestion that it can be used as an adjective as proposed in your example sentence.
Merriam-Webster's Eleventh Collegiate Dictionary (2003) gives one reasonably audience-free definition of encore as a noun (definition 2b below):
Meaning 2b applies to instances like this one: In 2004, after painstaking effort, Russian and U.S. scientists succeed in briefly synthesizing ununpentium. A friend of one of the scientists asks, "What are you going to do for an encore?" The figurative reference is to performing before an audience, but the question itself is about topping a remarkable achievement.
"The lecturer gave an encore presentation" sounds fine to me, by the way. Google Ngram Viewer results find the phrase "encore performance" as far back as the 1870s. A typical example (from 1892):
In such phrases, encore functions as an adjective and simply means "repeat." Merriam-Webster's doesn't endorse encore as an adjective, but that doesn't mean that people don't commonly use it as one, at least in the United States.
FOLLOW-UP: As for the history of "encore presentation," the phrase seems to have become fairly well established by the 1970s: Google Ngram Viewer search results return at least 18 unique instances of it between 1966 and 1979 in published works. The three earliest instances that Ngram Viewer finds date to 1945, 1955, and 1962.
From Bookbinding & Book Production, Volumes 42–43 (1945):
From Musical Courier, Volumes 151–152 (1955):
From Autoharp, Issues 11–20 (1962):
FURTHER FOLLOW-UP: In the period from 1969 to 1979, "encore presentation" came into more frequent use in connection with lectures, seminars, and public addresses. Ngram Viewer notes the following instances:
"Adding up the results after the last of the 240 visitors had left, Friden [a manufacturer of electronic calculators] officials said the $46,500 in immediate and projected sales justified an encore presentation" (International Commerce, 1970). "An encore presentation will be held at the New York Hilton on Monday, October 18th, again featuring speakers with varying points of view, a probing press panel and topics of current interest." (FACS Forum, 1975). "Next month, an NPL encore presentation will entertain Van Camp brokers vacationing at company expense on the isle of Martinique." (The Executive of Los Angeles, 1978). "A private trade show in the trucking industry, the Mack Trucks 'Super Spectacular,' was a success in its encore presentation in the New Orleans Superdome" (Industrial Marketing, 1979). "The seminar, an encore presentation from Chicago's June CES, drew enthusiastic response when Berman pointed out weak, self-defeating sales practices and the overuse of the phrase 'Can I help you?'" (Billboard, 1979).
The most common circumstance where "encore presentation" appears during the 1970s is in connection with repeat showings of TV shoes, as in this instance from The Pennsylvania Connection (1979):
Evidently, broadcasters decided that "encore presentation" would sound more appealing to potential viewers than "rerun."
Ngram Viewer didn't return any matches for "encore lecture," "encore demonstration," "encore address," or "encore seminar," suggesting that few encore + noun combinations are in common use in publishing. However, it did find quite a few for "encore appearance," from as far back as 1927.