I think there are two aspects to your question:

1) Implementation of a Flat Scenario

This can be done with nova network using Flat/FlatDHCP modes (probably just Flat since you want to use your own DHCP). In Quantum, this can be achieve creating layer-2 only networks with provider mappings. A provider mapping will explicitly connect a Quantum network to a specific physical network (e.g.: vlan 234 on eth1); in this case no subnet/router should be create at all

2) Building a typical quantum L2/L3 topology, but without forcing SNAT at the router. This not currently possible, although we are working on it: https://review.openstack.org/#/c/25525/

However, this would still make quite problematic to use your own DHCP server. If you are using an external DHCP server, it is likely you want to bypass Quantum's IPAM. This is not currently possible, even if there are people aiming at achieving this: https://blueprints.launchpad.net/quantum/+spec/configurable-ip-allocation