I would rather think of allowing a small piece of multi-master operation by allowing unsolicited messages from the slaves. As in any multi-master systems, possible collisions have to be handled.
I don't clearly see the point why you don't want to poll the devices, by the ways. Probably it's still the most economic way. If you want to reduce the communication amount, the master can signal a time window for optional answers, that is only utilized in case of pending messages.