Inner Frame Mounted Carrying Bag I've been thinking for awhile that having a bag on the inside of the frame of the bike would be a good place to store a few small things, such as a camelback bladder, shirt, etc. What I mean is a bag mounted where the yellow triangle is in this pic. For all I know, it's a common thing in bike shops, but I haven't been to one in a few years (unless you count Canadian Tire) and I've never seen anyone with something like it.

Would it be a good idea? Does it exist? If so, where could I get one?
There are many available.  Here is one example: http://www.ebikestop.com/axiom_cascade_frame_pack_black-BG6638.php

Attaches to the seat tube and the top tube, providing a triangular storage space.  This one also is padded to aid with portaging (if you have a big enough triangle).  

I know some people who tour with home-made frame bags that take up the entire triangle.

These bags are a great idea because they keep your load close to your center of gravity.