@jhp: try reversing the statements (move the '!" from the IP addresses to the variable).
Code:
direct_servers="{ 172.26.0.24, 172.26.0.32, 172.26.0.39, 172.26.0.41 }"
rdr on $int_if inet proto tcp from $int_if:network to ! $direct_servers port www -> 127.0.0.1 port 3128