So first, those aren’t examples of postpositive adjectives. A postpositive adjective is one that occurs after the head noun in a noun phrase: “something unusual,” “the money available,” “attorney general,” etc.
In “All men are created equal,” “(are) created” is a complex-transitive verb and “equal” is a predicative complement (PC). In complex-transitive constructions, the PC describes the direct object of the verb (or in the case of a passive voice sentence like the above, the subject of the verb).
Let’s look at the active version of the “all men” example:
“They create all men equal.”
“All men” is the direct object of “create,” and “equal” is the PC that describes “all men.”
Another example:
“They elected her president.”
“Elected” is the complex-transitive verb, “her” is the direct object, and “president” is the PC (a noun this time) that describes/applies to “her.”
Note how, semantically, the verbs in each example seem to have the ability to confer the state/thing expressed by the PC onto the object: you can create people such that they are equal, and you can elect people to the position of president.
Only a limited number of verbs can operate this way, and “discard” is not one of them:
“They discarded them useless.”
This doesn’t work because “discard” doesn’t have the ability to confer the meaning of “useless” onto the object “them.” That is, the newspapers didn’t become useless via or during the act of discarding - they were just useless and discarded, or discarded because they were useless. Contrast with:
“They rendered them useless.”
Here, the action of “rendered” conveys the state of “useless” upon “them.”
This is a difficult concept to explain/grasp, so let me know if you need further clarification.
Some other complex-transitive verbs are: “make/turn/drive/appoint/believe/presume/judge/pronounce/imagine/crown/vote,” and there are quite a few more, but it’s still a relatively small proportion of English verbs.
The comma in, “They discarded them, useless” works okay (though this may be considered non-standard/literary/informal) because it implies that you’re just describing the newspapers as useless, after stating what was done to them - you’re not trying to connect the meanings of “discarded” and “useless.”