That describes exactly my problem. I want to know which is the non-shifted key for e.g. !. As you pointed out, this is different between keyboards (1 for US and German keyboards, §(?) for Czech keyboards…).
And I have to do that from a given char (or string).
For now I have done some hacked solution which works in most common cases. But this cannot be the final one. I think SDL should have all the required information internally, it just has to be made accessible via an interface.