Yesterday I received Canon's Conversion Lens Adapter for my A590IS. I planned on lengthening the tube with some homemade supplies, to make a hands-free tube adapter, as recommended by local Kowa digiscoper Don Morgan. While I still plan on doing this, I noticed that when I fasten the adapter to the camera, I can hand-hold the camera+adapter firmly against the scope's rubber eyecup, which happens to place the camera lens at just the right distance from the eyepiece for digiscoping!
I headed out to Sandy Pt this afternoon in hopes of testing this out in the field. Sure enough, I was able to produce a vignette-free image without having to max out the camera's zoom. This is still a form of hand-holding, so getting crystal-clear photos will be a challenge, if not impossible, due to the shake produced by holding the camera in your hands and pressing the shutter-release.
The following photos are unaltered (uncropped, unsharpened) except for being compressed, and show that decent vignette-free photos can be taken with this method.
Not a hint of vignetting, and the zoom is not maxed out in either photo. It looks like I'll be happy with this method until I'm able to make a hands-free adapter, in hopes of being able to take some print-quality images in the future. But that type of result may ultimately require a professional adapter and a higher-end camera. After just throwing down $2700 for a scope, I don't want to think about that quite yet...