So, my personal setup is such that whenever an application needs my password I
have it call a little shell script. This way my password isn't on the disk in
plain text but is still acessible.

For darcs send, I needed to configure ssmtp, and a little work allowed me to
alias 'darcs send' to something which would correctly send an email without
requiring my password in /etc/ssmtp/ssmtp.conf. It requires a way to specify the
command darcs actually runs. Fortunately, there is a command to do exactly this,
--sendmail-command=, so I could write 
 darcs send --sendmail-command="torify ssmtp -au `gpg-address` -ap
`gpg-password` %t %<"

However, suppose I want to *sign* something with my private key, like I do all
my other emails? --sign doesn't work, because there's no way to tell it to use
my gpg-password shell script to get the passphrase for the key, and the
modifications to one's gpg.conf suggested in the manual both don't seem to work
for me and also seem kind of insecure. So I would personally like a
--gpg-command=, which works just like --sendmail-command (with different
escapes, obviously). That would let me solve my problem; I know it can be done
as I do a very similar thing in mutt - it just needs to be done.