I think your worries over walled gardens are over cautious. These sorts of tools (if you choose the right ones!) all produce RSS and are driven through a browser. Data no longer sits in a database hidden behind an opaque data access layer, it’s available as RSS and URLs, and can be linked too. To really make this stuff fly in an organisation you need an aggregation tool to close the loop.
We were lucky in the BBC to be given our head and fully subscribed to the old adage ‘it’s better to ask for forgiveness than permission’ (disclosure: I built and managed the systems in the BBC with Euan).
The trouble with top down is that the same old suspects start seeing it as part of their territory; Internal Comms decides there’s a message to be delivered, HR plan a branding exercise and start talking about rolling blogging into your annual performance review and so on. All these associations are guaranteed to be received with suspicion and hostility by staff who just want systems to help them do their job. We saw our job as creating the right initial conditions for this stuff to work, and then gardening like crazy to allow the right ecology to establish. Once you get a senior exec blogging, then you have the right sort of management buy-in.