You should be able to use something like:
Mlocal, P=/usr/local/lib/mh/slocal, F=SPDFMlns, R=20, S=10,
A=slocal -sender $g -user $u
Check the P= operand. Also check the R= and S=, which you should just
copy from your current local mailer definition.
Note that this assumes your sendmail runs suid root. The S flag in the
F= string passes this suid privilege to slocal when called by sendmail,
which allows slocal to change its identity to the recipient. I advise
against making slocal itself suid to root.
Slocal does not return sendmail-style return codes so any delivery
problems will cause mail to be returned instead of defered. If this
is not what you want, you may want to put a shell script wrapper
around slocal similar to the following.
Jeff
#!/bin/sh
/usr/local/lib/mh/slocal ${*} || exit 75