Hydration Pack with external pocket

No help with the hydration pack but I have had good success with holding gel packs by slipping a couple/few under the bottom of my shorts. With them being around the thigh area they are very easy to access while riding. They are slim, small and soft enough that once riding I forget they are there until I need one.

yeah i do that, along the taping them to my stem. Not a huge deal for xc races.

But 100 milers, I would like to carry more than what I can slipping them under my shorts

Wingnut makes a few different ones- I got the Splitback earlier in the year and I will never use anything else. Not only is it more comfortable than a "normal" pack, it's incredibly well made, lighter than most other packs, and the side pockets are large and really easy to access while you are riding (I even manage to fish things out of there with winter gloves on).