I saw your post in your blog and I am going to use it tonight. I do not use 2 ethernet ports although this is probably better. I only have a DD-WRT router connected to a Speedtouch 585v7 (as an ADSL modem in bridge mode) and my AMPRnet Gateway in a LinuxMint Box with one ethernet port and 4 radio ports in it's 4 x RS232 adaptors.
In case your DD-WRT uses PPPoE to "dial" an Internet connection via eth0, you should have a ppp0 interface which will receive your public dynamic IP (or if you're unlucky and your ISP employs CGN, you will receive a private IP and things probably just won't work).
On the DD-WRT ppp0 will be your (virtual) WAN port and eth0 will be your LAN port.
The eth0 on your DD-WRT and the eth0 on your Linuxbox should be able to use an IP address within 44/8 without issues. You will just have to make sure that packets from within your LAN which should go via the IPIP full mesh must use the LinuxMint Box as default gateway and not the DD-WRT, while all other boxes (not need for AMPRnet access) and your LinuxMint box must use the DD-WRT as default gateway. But you are free to set this up as you prefer.
73 de Marc, LX1DUC