Mac OS X - network interface preference defined by host domain
As has been pointed out in the comments, using DNS names for traffic distribution isn't possible (in your scenario). You need to provide local routing as desired and this needs to use IP addresses or subnets.

What you can do is set up the default gateway on the wi-fi NIC or lower the metric for wi-fi - if you want to stick to the all-DHCP setup you'll need to change this on the DHCP server. This takes care of using wi-fi as default network connection.

Then, you set up specific, local routes for the "Ethernet services'" IP addresses that have lower metrics for the Ethernet NIC with route add.