yeah I have a Thermoback (desert camo, 3 ltr, no pockets) just use it for races when I don't want to carry much.
I never fill the bladder, so yes there's room for tube, pump & multitool tucked in with the bladder, however access is poor – emergencies only!
There are hooks and eyelets on the outside, so it's easy to strap a waterproof to them, but if you want to do that sort of thing I'd get a bigger bag really.
There's no padding of any kind on the back – just sweaty fabric, and there's no waist strap. I like the external filler though, and mine has an insulated water pipe and swivelling bite valve, which are nice touches.
In retrospect, for the price difference you may as well get one of the basic bike-specific Camelbacks, which have at least a small tool pocket and padded back.