I too have required DD-WRT for a project on these mini routers. I have at least one of each model of the GL mini routers and have found the only truly models supported are the original GL.iNet6416 and GL-AR150. I did want to use the 300 series preferably and found that although there is documentation for DD-WRT on GL-MT300N and GL-MT300A. DD-WRT does work on these models but I have issues with wireless client and repeater. This I found not the issue of these devices but I have also tried with other models with the MTK76XX chipset and also works but I still have the same issues as it seems this is also an issue with the original Buffalo 300HP2 which the firmware is based on. I am currently working on modding DD-WRT firmware on the AR300M but without much success as yet. The hope on this device is to have dual boot. DD-WRT on NOR and OPENWRT on NAND.
My requirement for DD-WRT is a third party application installed on DD-WRT and also Tomato but unfortunately is not compatible with OPENWRT.