Zandr just mentioned that he wished for this feature.  (He wasn't previously
aware of this ticket or of the fact that other people have wished for the same
thing.)

Currently we have two candidate user interfaces for this.  One is that there is
a new interactive command available in the "darcs record" prompt.  The other is
that you can drop into a text editor to configure the hunks how you like before
going on with the record.

I think the former should be attempted first to see if it is sufficient, because
it feels simpler to learn and use than the latter.  My current proposal is as
follows:

Add a command to the 'darcs record' prompt which you get after it displays the
next hunk.  Currently your choices are record, don't record, don't care, skip
the rest of the file, record all of the rest of the file, etc..  Add one more
option: split this hunk into two equal-sized pieces.  If you choose that
command, then now you are no longer looking at the original hunk, you are now
looking at the first half of it, and you are in the exact same mode -- ready to
choose record, don't-record, split-this-hunk-in-two, etc.

My intuition is that this interface, while being extremely simple to learn and
use, would suffice for 90% of the use cases.