I currently manage a multisite installation with 7 member blogs. We need to produce a combined RSS feed of news from all 7. Currently we use Feedwordpress to syndicate 6 blogs into blog number 7. This isn’t working out because we’re duplicating content in the db.

Ideally I need a cached feed to reduce server load (the feed is subscribed to via high-traffic portals).

What’s the best way of doing this? I’ve considered Yahoo Pipes, but how about using WordPress transients to cache a query across all member blogs, and somehow publishing this as RSS?

JSON could be an option, but RSS is probably preferred.