Following the diacritics tutorial, I’ve added U+01F0 LATIN SMALL LETTER J WITH CARON. Isn’t there a capital letter j-caron? Can’t seem to find it’s unicode. Should I add one without the code? Also, what about small caps and smcp/c2sc features?
I would include the uppercase version even though it is not in Unicode. The listing for the lowercase says it is used in IPA and “many languages” but I think that is a bit misleading unless one is counting transliteration. The only actual language I know of which uses it as a part of their alphabet is Wakhi, native to Afghanistan, China, Pakistan, Tajikistan and Russia. Most uses is for transliteration of Cyrillic languages.
To support the Wakhi language you would also need to include J_caroncomb_dotbelowcomb and j_caroncomb_dotbelowcomb as well as several other glyphs you may not have included, according to www.geonames.de.
As a general rule of thumb for my fonts, if I have a precomposed lowercase glyph with an accent I always include the uppercase for user convenience regardless of Unicode encoding.
I don’t think you need a precomposed uppercase if there are no official casing rules. You can’t type it directly so you need to type a J and caroncomb and rely on mark positioning to move it to the right place.
Being able to type it has nothing to do with whether it is a single Unicode character or a sequence of Unicode characters, or whether it is a single or multiple glyphs properly positioned in the font.
Keyboard layouts can do one-key-to-character-sequence just like they can do key-sequence-to-one-character and key-sequence-to-character-sequence.
You don’t have to have a single glyph for J+caroncomb <004A 030C> (if you have the right features in place) but you can if you want.