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