i've tested out slaking down clay that's fired up to around quartz inversion - to some degree it does break down in water, but not all the way like greenware does, but ultimately it does kinda slake down i guess (at least at the temp my clay was fired to). the items had about the same strength as it would if it was regular greenware, so i'm not really sure if you'll get the strength you want to achieve by trying to fire the clay --- but every clay is different, and a hundred degrees in either direction could mean failure or success, so testing is definitely recommended.
My best guess is that you may have to add some sort of binder in the clay to help give strength, yet still decompose. Some of my immediate thoughts go toward clays with higher green strength - stuff like bentonites or ball clays. Then you have tons of organic binders that can be added into the clay - stuff like CMC, PVA, starches (aren't water-friendly golf balls made from cornstarch?), beeswax or paraffin, shellac, etc.
What about encapsulate your regular clay body with something that'll resist water for a while (like wax or shellac), but will break down eventually enough to let water in and slake the clay? You'd also be able to pigment that medium if you want color on your finished urn.