There is no way to "unlock" DLC or XBLA on a DVD-tray-flashed console. Only JTAG's can do that.
To get DLC working on a DVD-tray-flashed console... I'm not 100% sure.
I know that with my flashed Xbox, I purchased DLC from Live, and now that it is banned I can still use it with the account that purchased it.
It would be worth a shot to try this:
Using a Live-enabled console, sign-in with a Live account, buy DLC, download it to a USB/HDD attached via USB, then take the DLC and account that purchased it, and transfer it over to the DVD-tray-flashed console via NXE's 'Transfer Content' option under Memory in System Settings. Still, there might be a problem with licences not transferring.
Someone else might need to shed more light on getting DLC and XBLA to work on a flashed console.
(Actually, you can buy XBLA game packs as retail DVD games. Same game, just comes on different media.)