Thanks for the clarification, Wolfgang.

Am 20.04.2016 um 16:25 schrieb Wolfgang Schuster:
> [...]
>> I was looking for a general way how to substitute one glyph with 
>> another depending on context, thus I can't see how there are 
>> different problems in my post? But it's good to hear that others have 
>> thought at least of a partial solution.
>>
>> Is my guess right that normally the author of a font ought to provide 
>> otf features that can be switched on and off to get one result 
>> instead of the other? That area seems to be utter chaos in the 
>> Calluna fonts.
> The output of \sc to produce small capitals depends on the font and 
> it’s the font designers job to provide a small ẞ (he could use 
> stilistic sets to let you choose between ß and ẞ).

Well, there's the problem, I think. Jos Buivenga, aka exljbris, 
Calluna's font designer, has done a superb job on the graphics (in my 
eyes), but his stylistic sets or otf features seem to be full of holes, 
as far as I understand them, which is perhaps not very far.

What I'm looking for is a way on the TeX, LuaTeX or ConTeXt level to 
define "environments" ('scuse my LaTeX slang) within which certain glyph 
substitutions happen. Then I could kind of redefine a thing like \sc to 
use the smallcaps ß and switch to smallcaps lining figures, if 
available, or, otherwise, scaled-down uppercase lining figures. I have 
still other fish to fry in this context, but a general hint where to 
look for something like this would be helpful.

Thanks a lot so far -- Thomas