Yeah - when I made the suggestion I was picturing, like the PP said, just basically cut off the arms of the sweater and sew of those holes and the neck hole (or just use the body of the sweater, depending on the size you need. I usually felt before cutting, but the person who suggested felting after had a good point. Maybe I'll try that technique sometime.
What I was wondering about doing for the drawstring is to crochet a long tie, or use some ribbon or something as a tie, then weave it in and out, like you would for a tie on longies. Seems easier than fussing with a drawstring, but i don't know if you would cause too much wear over time. The zipper is a good idea, but then again, I don't really love sewing zippers.
Again, I've never seen one of these and never tried it, but I want to hear how it works when one of you actually makes one!
The only thing I wondered about functionally was the issue with lanolizing, how often, etc - since I assume the bag will likely need to be cleaned more often than a soaker might (especially if it's regularly holding poopy diapers).
Let's see pics and updates!