Tried to install DD-WRT today as a router for my virtual environment in Hyper-V.
Worked very well and does not require very much resources so sharing some very simple steps:

1. Download Phydiskwrite bundle with GUI from http://m0n0.ch/wall/physdiskwrite.php
2. Download Community version of DD-WRT from http://www.dd-wrt.com/routerdb/de/download/X86/X86///dd-wrt_public_vga.image/3744
3. Create and mount an empty VHD of 50mb from within the DiskManagement MMC console.
4. Start the PhydiskwriteGUI with admin privs. Write the image file to the virtual disk (make sure to select right disk!!) by right clicking the virtual disk and selecting to write image. When you browse for image you must select *.* as filter as it is by default setup for *.img not *.image
5. Unmount the VHD
6. Create a virtual machine with 1 cpu, 64mb ram, attach the VHD and two legacy network adapters (1 to external and one to the host-internal network)
7. Start the VM. It will now have a dhcp running so connect to 192.168.1.1 and configure.
8. Done!