I am doing this on my Zeroshell setup but am using ETH00 to do it (ETH01 is my other ISP with ETH02, ETH03 and ETH04 being internal networks).
My DSL modem/router uses its default 192.168.1.1/24 address for management. I have configured Zeroshell’s ETH00 as 192.168.1.10/24 with NAT so you can access the DSL modem/router at 192.168.1.1.
On the DSL modem/router I have configured it to bridge ATM to Ethernet.
On Zeroshell I have created PPP0 and set it up to use ETH00 for PPPoE.
Zeroshell handles the PPP session with the ISP. I can access the DSL modem via a web browser for any management issues.
I believe that the original question asked about an equivalent setup, so yes it is possible.