You can do it by setting CAP_NET_BIND_SERVICE capability to the application in order to allow non-root service to listen on privileged ports. This can be done with setcap which uses setxattr to change security.capability extended attributes of the file.

For Gentoo:

• Install libcap

  USE=pam emerge -av libcap

• Use setcap to set CAP_NET_BIND_SERVICE capability to the service binary

  setcap cap_net_bind_service=+ep /path/to/service

You may also want to edit /etc/security/capability.conf file to restrict a specific user for the capability (you need to enable pam USE flag for libcap).

HTH

--rd

P/S: you can also bind the service on a non-privileged port (ex: 8080) and use iptables to do port forwarding from port 80 to 8080.