>In cold water, it takes about 30-40 minutes to rehydrate.
Actually, that's a good point in general. Even Mountain-House type foods will eventually rehydrate on warm days. Eat cold foods for breakfast and lunch, put unheated water in the MH bag, reseal it, stick it in your BP's back or side sleeve (where it will get some sun, preferably), and walk for six hours or so. (The temptation to stick the dinner in my pants to warm up has sometimes been almost overwhelming, but I have deferred.) ;-D
The food sometimes ends up a little crunchy, but, heck, we're not on the trail to eat or we wouldn't be taking MH dinners along.