The process itself has to call setuid(2). You should also investigate running it inside chroot(8) if you arent already. As far as I know there is no way for root to change the uid of another process.

If the reason you are running it as root is to bind ports I'd suggest running it as a normal user on a higher port and using ipfw(8) on OS X to forward port 80/443/etc to the higher port:

http://support.crashplanpro.com/doku.php/recipe/forward_port_443_to_pro_server_on_mac_osx

The process itself has to call setuid(2). You should also investigate running it inside chroot(8) if you aren't already. As far as I know, there is no way for root to change the uid of another process.

If the reason you are running it as root is to bind ports, I'd suggest running it as a normal user on a higher port and using ipfw(8) on OS X to forward port 80/443/etc to the higher port:

http://support.crashplanpro.com/doku.php/recipe/forward_port_443_to_pro_server_on_mac_osx