I was able to get this working using the ifconfig and pfctl commands on Mac 10.10.2. With the following approach I'm successfully mapping 127.0.0.1:3000 to mydomain.com locally on my machine.

MAC OS X: mapping 127.0.0.1:3000 to mydomain.com locally on my machine

In your command line enter the following two commands to forward connections to 127.0.0.1:3000 to 10.0.0.1:

sudo ifconfig lo0 10.0.0.1 alias
echo "rdr pass on lo0 inet proto tcp from any to 10.0.0.1 port 80 -> 127.0.0.1 port 3000" | sudo pfctl -ef -
Then edit your /etc/hosts or /private/etc/hosts file and add the following line to map your domain to 10.0.0.1.

10.0.0.1 mydomain.com
After you save your hosts file flush your local DNS:

sudo discoveryutil udnsflushcaches
Now open mydomain.com in a browser and you'll be seeing the server hosted on your localhost port (i.e. 127.0.0.1:3000). Basically this process maps an <ip>:<port> to a new <ip> so that you can then map a host that IP.