I’d be very interested in seeing your workaround. I, too am looking for some way out of the straitjacket of this potentially exciting feature. See https://discuss.gradle.org/t/restriction-that-plugin-versions-must-be-string-literals-is-onerous/18152.
On the subject of settings.gradle, are you aware of any clear description of what kinds of things can or cannot be placed there? If I knew this, I might find some workarounds.