test.tex

\documentclass{article}%

\usepackage[utf8]{inputenc}%
\usepackage[LY1,TS1,T1]{fontenc}%
\usepackage{unicodedomino}%

\makeatletter%
\message{:expect 48:\decode@UTFviii0\relax:}%
\message{:expect 228:\decode@UTFviii ä\relax:}%
\message{:expect 8364:\decode@UTFviii €\relax:}%
\message{:expect 128169:\decode@UTFviii 💩\relax:}%
\edef\temp{\decode@UTFviii1\relax}%
\message{:detok=\expandafter\detokenize\expandafter{\temp}:\expandafter\detokenize\expandafter{\decode@UTFviii2\relax}:}%
\makeatother%

\begin{document}

foo

U+009F Ÿ

U+05D0 א

U+3090 ゐ

U-0001F4A9 💩

U-0010FEDC 􏻜

bar

\end{document}