I don't think that can be done out of the box. Atleast, I don't see it
anywhere, but I could be mistaken.
However, if you are willing to recompile your system, perhaps changing
references from RFC1035_TYPE_A to RFC1035_TYPE_PTR in tcpd/tcpd.c
docheckblocklist() might do the trick. It looks like it would cause
-block on couriertcpd (use BLACKLISTS in etc/esmtpd) to do reverse
No. This is the lookup itself, not what's being looked up.