You're right that you won't get much detail out of an ED80 at high magnifications. Extension tubes work well in Newtonians, not sure how the ED80 would cope. It's might be an easy thing to try though (without a diagonal). GSO make an inexpensive 5x barlow that's supposed to be decent. Bintel sells it under their own brand name:
http://www.bintel.com.au/Eyepieces-a...oductview.aspx
(Andrews is the other dealer I know of but seems to be sold out atm.)
Still I don't think there is much point spending even that much unless you intend to use it in the future with a larger scope. The ED80 is really best for widefield DSO photography. Or lunar or solar.
Look at it this way: The resolution of an 80mm scope is about 1.5 arc sec. The angular size of Jupiter is about 40 arc sec. What this means that if Jupter is about 50 pixels in diameter on your camera's sensor, the pixel resolution is already nearly double the scope's resolution. Making the image bigger will just give you a bigger blob with no more detail. I appreciate that it's often convenient to oversample, but past a certain point you could achieve the same result by scaling up the images digitally.
On stacking barlows: it can work but visually at least I never found it all that great. EP projection can work well provided you can get a good alignment between the optical axes of the EP and camera.
