Mooring to stern and raising the chute in light air should not be a problem.
It is not a particularly good way to practice but it is pretty good for
learing the very basics. As a way of debugging hardware and setup, it makes
a lot of sense. There is no concern for navigation, sail set etc. that come
into play while sailing. No one even has to touch the tiller. I suspect such
a situation would not be with a full crew who want to enjoy the water and
would get bored with on-the-water spinnaker practice. As for the boat
turning around, I seriously doubt it. Before a spinnaker could force the bow
upwind, it would collapse. A jib would have more chance as it has a rigid
leading edge but in my experience, it has too many other forces to contend
with. I have found some boats that are not stable when moored by the bow. I
had a Star that was just such a boat. A slight shift in wind direction and
it would try to pass the mooring. The line would then pull the bow and it
would, in effect, tack and keep going back and forth. This was without any
sails or people on board. Mooring downwind had no problems.
Dave.