"Based" here is either the past/passive participle of the verb base used in the passive construction "BE + past participle", or an adjective derived from the past participle (a departicipial adjective—also known as a "participial adjective"). "The film is based on a novel by Pat Conroy" means approximately the same thing as "[The creators of the film] based the film on a novel by Pat Conroy".
Running through a few tests for establishing adjective-hood, I couldn't find a totally clear indication of whether "based" is an adjective or a participle in this context1.
But in either case, it's not really a case of using -ed to mark a verb form indicating the current state. The thing that indicates present state in this sentence is mainly the auxiliary "is".
Tests for adjective-hood in English
1. I go over some of these tests in my answer to a question on ELU (which is partly based on BillJ's answer on the same page).
In my judgement, "based" in this sentence doesn't pass the "very" test.
The "very" test for distinguishing departicipial adjectives from participles is based on the commonly accepted fact that a verb cannot be modified by the word "very" (in a comment below, Greg Lee gave the following example showing how we can see that "very" obviously doesn't work with "based" when it is used as a finite verb: "*The author very based the film on a novel"). Therefore, if the word "based" in your sentence can be modified by the word "very", we can be sure that it is not a verb (and so we would assume that it is a departicipial adjective). But in fact, we can't modify "based" with "very" in your sentence:
- *"the film is very based on a novel by Pat Conroy"
Not all adjectives can be modified by "very", so failing the "very" test doesn't prove that something isn't an adjective. Thus, the "very" test is inconclusive.
"Based" in this context also doesn't seem to past the "un-" test, which establishes that something is an adjective (keeping in mind that the adjective prefix "un-" functions as a negative prefix, unlike the "reversative" verb prefix "un-"):
- *"the film is unbased on a novel by Pat Conroy"
As with the "very" test, this only fails to show that "based" is an adjective here: it doesn't show that "based" is not an adjective, because not all adjectives can be prefixed with un-. (In fact, based on the results of a Google search, the word based followed by a prepositional phrase starting in on can be prefixed with un- in some other sentences, so it seems that based is an adjective sometimes; but I'm not sure what the criteria are for when it is and isn't possible to use "unbased on..."--it may be that only some speakers use this construction.)
"Based" in "based on a novel by Pat Conroy" might be able to pass the "seem" test, which is apparently considered to establish that something is not a verb/participle:
- ?"the film seems based on a novel by Pat Conroy"
Compare the use of the in my view semantically similar "seems based on a true story" in the following quote:
One test out of three doesn't seem quite convincing to me; also, even if it is the case that "based" is an adjective in some sentences, it doesn't necessarily mean that it is an adjective in the original sentence that you brought up. That said, it's true that the sentence has a stative rather than a dynamic meaning, and "stative" passives often are analyzed as containing departicipial adjectives rather than participles.
One interesting thing about based on... (or unbased on...) as an adjective is that the prepositional phrase starting with "on" seems to be an obligatory complement. According to Rodney Huddleston's Introduction to the Grammar of English, "very few adjectives take obligatory complements" (p. 305)--Huddleston gives the example fond: we can say "Sally is fond of sweets" but not *"Sally is fond". Other examples are devoid of and lo(a)th to.