\def\scaleglue#1#2#3{% #1 is a factor, #2 is a glue specification, #3 is a glue register
#3=#1\dimexpr#2\relax
plus \strippt\dimexpr#1\gluestretch#2\relax
\ifcase\gluestretchorder#2 pt\or fil\or fill\or filll\fi
minus \strippt\dimexpr#1\glueshrink#2\relax
\ifcase\glueshrinkorder#2 pt\or fil\or fill\or filll\fi
}