I don't have any such code handy, but it seems like one could put smarts
into the cronjob to either sleep or abort if you are active or have been
recently (for your definition of that). Basically, look at the output from
the "IDLE" column of the "w" or something similar. Since that looks at
terminal idleness, and nmh is all CLI based, if you are using a web browser
or the like, there won't be a conflict. If you are using mh-e, exmh, or the
like, there could be as I am not sure how they show up. There is still a
corner case where the cronjob starts and then you start doing something in
nmh of course.
Right, I mean ... that's probably sufficient. But there aren't any
universal solutions. If you want to wrap every nmh command in a
global lock, that's also an option.
--Ken