[Jack]

Interesting idea, and I'm sure it can be made to work.
Get the seating depth by trying a dummy round in the chamber of your barrel - you'll have to see how much of the shot capsule can protrude from the case mouth. You may not be able to seat the capsule out too far before it contacts the lands.
Use the smallest shot you can get your hands on. I generally use #9. No point in using big shot- you're effective range in a rifled barrel will be 10-15 feet.
As for a powder charge, I would call up Speer and ask them. My guess is that the powder charges recommended for the 38 Special are going to be too high. My guess is that you'll have to seat the capsule fairly deeply in the Super case, and won't have a lot of room left for powder. BTW, high velocity is not something you want in a shot load. I've pattern tested quite a few in 38 Special, 44 Special, and 45 Auto- once the velocity gets much over 1,000 fps, the patterns start to show big holes in the center.
I'm assuming you taper crimp your usual loads. That probably would work on shot capsules, too, if you don't use a heavy crimp.
Let us know how it works out.