An extension  was ratified to embed a RequestedAttributes array in an AuthnRequest extension as a dynamic signal for attributes during SSO. We have the machinery to process this already in place so it's probably not a ton of work to support this. The expedient thing may be to just simulate the use of an AttributeConsumingService by perhaps manufacturing a fake one to embed in an AttributeConsumingServiceContext.
By spec, the use of an AttributeConsumingServiceIndex overrides this extension. If we extend the existing action bean that pulls out the index and have it mock up an AttributeConsumingService the support is likely automatic.