I've been driving myself nuts trying to recall in which catalog I have seen watering bags that were designed to be used with drip irrigation tubing or soaker hose. What I saw were basically 5-gallon bags that were suspended above the ground on stakes.

I think these things would be ideal for my raised beds and rain barrels. Any info on sources (or ideas about how I could jerryrig my own!) would be most welcome.

I'm forever driving myself nuts, so welcome to my Reality!
One source that may be worth looking into is junkfood emporiae! I used to use the lined "bladder" types of bags that contain the (sickly sweet) sodypop concentrate for fountain drinks as dispensed by the quazillions of gallons. In this case, it was a Harvey's up in Ontario (great flame broiled cheeseburgers, by the way) and while rinsing/flushing the syrup was a chore, the price was right at the time. Used a small dia. clear plastic flexible tube (box stores, cheap) to connect several bags and left the last bag on the "chain" open for overflow.
A bit of engineering but perhaps worth looking into - and a likely way to recycle at least some of the stuff that goes to landfill.

John
http://www.ecologicalsgardens.com/