Can subscribe by email trigger notifications from all sites on a multisite network

Hi,

I have a few multisite networks running and typically i place a subscribe by email widget in the footer. This is placed on a site by site basis.

I wondered though if there is an alternative method that would allow people to subscribe from any site and receive updates from all the sites (or more than just one) on the network.

My client case example is where I am using multisite to separate out the main sections of the site and run separate themes for a main site, /directory, /knowledge, /forum and /shop. To the end user these all appear to be the same site and they should be able to get notices about updates on any of these sections from one email subscription...without having to subscribe in 5 places.

Are there any options for this?

Thanks
Chris