In 06 I used the Cycoactive thing on my aerobars, for 07 I was playing around with something very similar to what you describe in a box like arrangement. I made it tiny - too tiny in fact. I've also used my Ortlieb bar bag with the great map case...
Here's a prototype I'm hoping to have some parts made for: (this dates from awhile ago, sent to some machine nerd friends... if I have time I'll make one for 08) (pardon the modeling, I have a clean version somewhere...)
going low tech, paper style. I've searched the web and the best I can do is map cases (already use one and have issues) and the bulky (heavy!) motorcycle roadbook holder.
So, I've been playing around with some sample cases and its just not working out... it would all be a hack job and not really work.
This is what I've come up with:
Acrylic tube 3" OD about 5.5" long.
Aluminum end caps with a gasket of some sort to keep out the weather.
Interior acrylic tube 2.5" OD (half a tube, used as a base for pulling the paper cue over).
2 interior aluminum rods to hold / wind the paper. (turned down on the ends, maybe to slide into nylon bushings)
Aluminum base inside to hold the ends together and serve as an LED mount. (this would be bolted to the interior end caps)
Room for a battery and LED for lighting this up at night (from within, using a red LED so as to not wipe out my night vision).
End cap oppositte knobs spins off on a threaded stud for easy no tools access / adjustment. (need gasket to keep out water)
End cap with knobs is fastened to interior aluminum base (both sides) to hold the inner works together.
4.25" (most cues are set up to print 4 per 'letter' sheet) paper rolls on one end, then over the interior tube and onto the other roll. I'll need some sort of drive belt between the 2 rods to sync both rollers, so I can go forward and back without screwing up the roll. (and work the thing from either knob)
I'm working out the mounting details next. I'll mount it centered on my bars, just in front of the stem. Probably some sort of bolt patter through the outer tube at the base. You simply loosen the aluminum end a bit to adjust the angle.
I also need to work out the paper clip attachment thing. I'm thinking some tape will work just fine... but was also considering a spring clip of some sort.
Mount from below, on something that will float just in front of the bar tops. I'll need something here too - but might be able to adapt something to a h-bar bag mount.
So, that was last year... I've rethought the whole thing and am hoping to make it from off the shelf parts. I have some of them on hand somewhere... as spring gets closer I'll see if it is worth it.