x-smcp-letterspace.tex

\input luaotfload.sty

\font \foo = file:Iwona-Regular.otf:+smcp;letterspace=1.25 at 42pt
\font \foo = file:MinionPro-Regular.otf:+smcp;letterspace=1.25 at 42pt

\foo It works!

\bye

Here's an example with LuaLaTeX

\documentclass{article}
\usepackage{microtype}
\usepackage{fontspec}

\setmainfont{Linux Libertine O}[
  Renderer=Basic
]

\begin{document}
\textls[50]{\scshape This\showtokens\expandafter{\fontname\font}}
\end{document}

The log file reports:

"LinuxLibertineO:mode=base;script=latn;language=DFLT;+tlig;+trep;+smcp;"+50ls

that doesn't seem well formed. However this is not the problem; when Renderer=Basic is omitted, the +smcp type is ignored though the terminal reports

"LinuxLibertineO:mode=node;script=latn;language=DFLT;+tlig;+trep;+smcp;"+50ls

In synthesis:

• the ligature Th is kept, which is awfully wrong, when Renderer=Basic is used, small caps are used for the rest;
• the letterspacing is maintained (and no Th ligature is formed) but no small caps