Trouble with contextuals in LuaLaTeX with fontspec - trying to get internal long Ss to work

Question

I have been trying to get a project of mine set with internal long Ss. Using fontforge, I have tried to set up the contextual (both [calt] and [cswh]) feature - but when I do, I get no results in the typesetting. I have gone the other direction and set all Ss to be long, but to have the regular one as a final [fina]. This almost works - however, it doesn't regard the symbol as "final" if followed by a punctuation mark.

Is there a way to investigate and alter what constitutes Word-Final?

It may be that I'm building the internal context wrong for [calt]. I'm make a single substitution of base glyph "s" and substitute glyph "s.long"; then doing a [calt] chaining substition: match "s" with lookahead and pointing it to the single substitution; and the same with match "s" lookahead "s".

Any suggestions on what else I should try?

Answer 1

(Probably not an answer you're looking for, but I do set my writings with internal long s, as e.g., in The Method of Physicke, and since this Q remains unanswered for more than a month…)

I use fontspec (of course) but in XeTeX not in LuaTeX.

I don't modify fonts (too many of them and most probably licenses don't allow) but have set a font mapping and respective hyphenation file. The source of mapping file (tex-text-PL-s.map) is like

U+0073 U+006F <> U+017F U+006F ; s -> long s o
U+0073 U+0070 <> U+017F U+0070 ; s -> long s p 
U+0073 U+0071 <> U+017F U+0071 ; s -> long s q
U+0073 U+0072 <> U+017F U+0072 ; s -> long s r
U+0073 U+0073 <> U+00DF ; ss -> long s s

and need to be compiled to a .tec file to be understood by XeTeX when […Mapping=tex-text-PL-s,…] font option is specified.