Still no answers, but there is a solution to the problem in part - after some 12 hours plaaying with SA.
To pass a single value using the standard SA registration form, create a stock dreamweaver recordset containing a single value.
Like select club_index, club_name from clubs where clubsession = 2
Then populate a menu with the single value within the recordset.
Bind the insert record field to the recordset field and you have done.
It looks like a drop down list but it only contains one value. Must be a better way but it will have to do for the moment.
If you need to register someone from the admin side, all that is is needed is copy the file and widen the scope of the recordset to include more options in the copy.
Still don't know how the SA hidden field is used. Should just be able to bind the session variable to it but ......?