I've created a set of configurations, one for each sub-project, and put them in a map, where each key was a configuration name, and the value was the name of the corresponding project. I then iterated over this map twice, once in the dependencies section:

dependencies {
    rpmConfigurations.forEach { (rpmConfiguration, subproject) ->
        rpmConfiguration(project(path = subproject, configuration = "dependencyJars"))
    }
}

and once in my file copy task:

val syncFiles by tasks.registering(Copy::class) {
    rpmConfigurations.keys.forEach { rpmConfiguration ->
        from(configurations[rpmConfiguration])
    }
    // ...
  }

This worked fine! Thanks again! 🙂