I gave a talk to the campus, attended by five hundred to a thousand people
"Attended by five hundred to a thousand people" is anchored to or modifies the object, noun phrase, "talk to the campus".
I gave a talk to the campus, attended by five hundred to a thousand people
"Attended by five hundred to a thousand people" is anchored to or modifies the object, noun phrase, "talk to the campus".
I gave a talk to the campus, [attended by five hundred to a thousand people].
The bracketed "attended" expression, a past participial clause, is not a modifier. The comma marks it as a supplement (an appendage), a non-modifying element presenting supplementary non-integrated content. It has "a talk to the campus" as its'anchor".
It's the equivalent of the finite relative clause in "I gave a talk to the campus, which was attended by five hundred to a thousand people".
has [the direct object] 'Bill' as anchor for both [participial] supplements (decreed by logic).
obviously now must have 'the park' as anchor.
is still acceptable, but the two supplements must be interpreted as having different anchors. Note the ABba {[anchor A] [anchor B] [appendage to/qualifier of B] [appendage to/qualifier of a]} structure:
..................
is unacceptable.
(1) I gave [NP a talk] to the campus, [VP attended by five hundred to a thousand people]. <OP's example>
Here, attended by five hundred to a thousand people forms a verb phrase (VP), and the VP is headed by attended (what you call a 'participle').
The VP attended by five hundred to a thousand people doesn't modify anything, but it merely is anchored to the NP a talk. Since the object of the verb gave is the NP a talk, neither attended nor the VP attended by five hundred to a thousand people is part of the object.
I notice @BillJ has raised this important question in the comment section:
What puzzles me is why the author wrote it as a supplementary clause, not a modifying integrated one, i.e. without the comma.
And I'd like to address the question for I for one think that omitting the comma renders the sentence ungrammatical. @BillJ suggests this in a comment about how to parse the version without the comma:
If it had been written as an integrated (defining) relative clause, the antecedent would be "talk to the campus". I don't think anyone would misconstrue this.
This would be parsed like (2):
(2) *I gave [NP a talk to the campus [VP attended by five hundred to a thousand people]].
But there's a problem with (2). Here, the prepositional phrase (PP) to the campus modifies the noun talk. But this is incorrect since the PP is supposed to be a complement of the verb gave. Therefore, the comma cannot be omitted from the OP's sentence.