At the risk or raining on your parade...
ANOpax,
Don't worry, I have an umbrella.
Thanks for the information. I didn't know about the Duo spinning restriction, which is ironic, considering I have 17 hours in it, flying dual cross-countries in the Southern Alps. However, since I wasn't going to be going solo, the issue of spinning and/or reading the handbook didn't come up. I do remember one stall "attempt", which was more "slow flight with high ROD" rather than a stall.
On the other hand, I have never even seen an ASK-21 or a DG-1000! However, I am intimately familiar with their handbooks, because I have just been teaching at a ground school, where my topic was weight and balance and why the CG limits are specified. Our club uses the ASK-21 as its "basic" trainer and they do have the spin weights and they routinely spin it.
I think the idea of an instructor surprising a student is not a very good instructional technique. If I wanted to demonstrate the DG-1000 behaviour that you describe, I would brief the student and then demonstrate it or if the student was reasonably competent, have him do it.
As an aside, I found the manuals for both gliders very poorly written and confusing. After looking at the originals, I concluded that it wasn't just a case of poor translation. In the case of the DG-1000, looking at weight and cg ranges specified in various places, it is hard to determine what is recommended versus being a certificated limit. The latest version of the ASK-21 handbook has an addendum, containing a USAF document about the spinning characteristics, because the USAF clearly felt that the manual was not satisfactory.