\input luaotfload.sty
\font\test={file:GenBasR.ttf:script=latn}
%font\test={name:AntykwaTorunska:script=latn}
\test ä\quad Ä

\test a\char"0308 %% -> combining dihaeresis
\quad A\char"0308 %% -> combining dihaeresis
\quad j\char"0323 %% -> combining dot below

\bye