\input{luaotfload.sty}
\font\4={file:xits-math.otf:mode=base;script=math} at 10pt
\textfont4=\4
$\hat{\mathchar"0161}$\par
$\hat{\mathchar"1161}$\par
$\hat{\mathchar"2161}$\par
$\hat{\mathchar"3161}$\par
$\hat{\mathchar"4161}$\par
$\hat{\mathchar"5161}$\par
$\hat{\mathchar"6161}$\par
$\hat{\mathchar"7161}$\par

\def\hat{\Umathaccent "7 "4 "0302 }
$\hat{\Umathchar"0"4"61}$\par
$\hat{\Umathchar"1"4"61}$\par
$\hat{\Umathchar"2"4"61}$\par
$\hat{\Umathchar"3"4"61}$\par
$\hat{\Umathchar"4"4"61}$\par
$\hat{\Umathchar"5"4"61}$\par
$\hat{\Umathchar"6"4"61}$\par
$\hat{\Umathchar"7"4"61}$\par
\bye
