Currently \primargs_get_mudimen:N actually grabs a muskip instead. Getting a mudimen (useful as an argument to \mkern) is much harder because there is no corresponding register. To do things properly requires implementing TeX's logic for that case, covering many complex nested situations such as \mkern\count\numexpr1+2+3\relax\gluetomu\glueexpr... However, it may be possible to be clever and grab many tokens at once: in the example here, first grab an integer, then find the unit.
Currently
\primargs_get_mudimen:Nactually grabs a muskip instead. Getting a mudimen (useful as an argument to\mkern) is much harder because there is no corresponding register. To do things properly requires implementing TeX's logic for that case, covering many complex nested situations such as\mkern\count\numexpr1+2+3\relax\gluetomu\glueexpr...However, it may be possible to be clever and grab many tokens at once: in the example here, first grab an integer, then find the unit.