I can instantiate a copy of Excel, and invoke other routines like
"MIN", but am not having any luck with "MDURATION" and, indeed, it
does not appear in the code completion dropdown for
WorksheetFunction.mDuration

Sounds like I'm out of luck.

Needless to say, I still have the option of pushing a formula into a
cell, pushing the arguments into other cells, excuting the formula
cell, and then picking off the result.

But that seems wrong somehow.... at least if there's a direct route
via .WorkSheetFunction...