This is probably something that should be added as an option. The JMS outbound adapter has a configuration attribute 'extract-payload' (default is true), so we could offer a similar attribute on the mongodb adapter. Feel free to open a New Feature issue: https://jira.springsource.org/browse/INT.
In the meantime, I suppose you could use a QueueChannel backed by a mongodb message store to distribute the objects, but it's a bit of an anti-pattern.
Gary P. Russell
Spring Integration Team
SpringSource, a division of VMware