Maybe the answer is so obvious I shouldn't need to ask the question, but here goes...
How can I put Metric holes in an Inch part, or vice-versa?
Looking for answers, all I see are references to Callout conversion errors, or advice on changing units of the part. I don't want to change the units, and I'll deal with any conversion errors as they arise.
I simply want to be able to use Inch or Metric threads and clearance holes, wherever I need them, without regard to the Units of the part or assembly I'm working in.
Just open the Threaded-Hole dialog and select the type of thread, be it Metric or Imperial, irrespective of what the model units are. Granted, the Threaded-Hole feature may default to the current Part units but that does not prevent you from selecting the thread class that you wish to use.
Thanks to both of you; as I said maybe the answer is too obvious!
With the settings box collapsed at the bottom of the dialog I just didn't notice it. (I'd recommend the dialog be reorganized to put the Settings panel (renamed to "Standard?") above the Form and Dimensions panel - making the workflow move from general to specific - but I can live with this now that I know the options are there.