The grammar of the original clause in question is sound:
such soldiers were heroes equipped, armed, and provided by the people of the king's land
As I analyze it, this is a simple, independent clause.
"Such soldiers" is the subject. "Were" is the copula. "Heroes" is the simple subject complement.
The noun "heroes" is modified by a complicated participial phrase. The phrase begins with a compound participle: "equipped, armed, and provided". Such soldiers were equipped heroes, such soldiers were armed heroes, and such soldiers were provided heroes.
Just as each participle can modify the noun "heroes" separately, they can modify it when compounded: Such soldiers were equipped, armed, and provided heroes.
"By the people of the king's land" is a prepositional phrase. It modifies the compound participle in the manner of a verb's adjunct. It serves as a sensible adjunct of each of the three participles.
You can paraphrase this clause by promoting the participial phrase to a relative clause:
such soldiers were heroes who were equipped, armed and provided by the people of the king's land
Some analytic frameworks assume that the relative clause is more fundamental, and that the participial phrase is derived through a process called whiz-deletion. Under such a framework the phrase is labeled a reduced clause. If that's the kind of framework that you understand, then the answer to your question is yes, whiz-deletion is permitted here. The "who were" is optional.
I prefer an analytic framework that assumes the participial phrase as more fundamental than a relative clause. The participial phrase requires no other label. If that's the kind of framework that you understand, then the answer to your question is yes, the participial phrase can modify the noun "heroes" directly. It doesn't require the mediation of a preposition or a verb.
That said, I also must say that TRomano's comment has merit.
such solders were equipped, armed and provided by the people of the king's land
The repetition of "heroes" isn't strictly necessary. The participial phrase itself can serve as an argument of the verb "were".