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Thanks for the suggestion, I hadn't thought to try that, but in this case it makes no difference. The following does seem to work, though it's not pretty:

configurations.all {
	resolutionStrategy.eachDependency { DependencyResolveDetails details ->
		if (details.requested.name == 'A-bom') {
			details.useVersion 'V2'
			details.because 'override for backport'
		}
	}
}