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You should try to use attributes to match the configurations. On the consumer side (where you collect things) you can do something like this:

configurations {
    myConfigurationConsume {
        canBeConsumed = false
        canBeResolved = true
    }
    attributes {
        attribute(Usage.USAGE_ATTRIBUTE, objects.named("my-usage"))
    }
}

rootProject.subprojects.forEach(pr -> 
    myConfigurationConsume project(pr.path)
)

And on the producer side you declare the attribute as well:

configurations {
    myConfiguration {
        canBeConsumed = true
        canBeResolved = false
    }
    attributes {
        attribute(Usage.USAGE_ATTRIBUTE, objects.named("my-usage"))
    }
    ...
}

Then when you resolve, you filter out the projects that fail to resolve because they do not have "myConfiguration" by using a lenient artifact view:

myConfigurationConsume.incoming.artifactView { lenient(true) }.files

That's awesome, thank you! I'm not sure that this solves my problem completely, though, because this build is complex in a not-so-good way 😕 I might not have a classpath with all the relevant source projects in the project where I need the refer to those files. This is a big and complex build, so I cannot easily be sure about that and even if it works now, that might not be true forever. So ideally, I'd like a way to collect all projects applying a certain plugin.

I think the "applying a certain plugin" is difficult without running into cross-project configuration (and timing) issues.

Looking at the code you posted above, it seems to me now like it would already do what you described. Is that correct?

Yes it should (haven't tested though).

Awesome, thx!