Camelback makes a hydration pack with a built in back protector. Its basically a thick dense foam pad. It gets the job done. I always wear a pack mtb'ing
If its warmth you're after, Thermerup heated insoles are pretty darn good. Rechargeable batteries, put them in whatever shoe you feel like wearing that day. The battery goes in a velcro band around your ankle. It can be mildly uncomfortable, you have to get it on the meat of your ankle vs...