There is a bit of experimentation to be done, not only to be able to access an optical drive at all but also to access it across a USB interface (unless that's what xyz has done) and then getting it mounted into the file system having plugged it in (be it automatic or manual). There may also be problems with different drives requiring different drivers.
Unless you feel like having a crack yourself, I think you had better regard this as a work-in-progress and be willing to test run the early versions of any packages that come forth (which is easy enough when you have the custom firmware plus an Internet connection).
I have to say though, once you do access photos on a DVD you might be pretty disappointed with the Humax's facilities to create a slide show and display them. I recommend you have a play with some photos on a hard drive and see if it will be worth the bother. Old TV shows are a different matter, it depends on their file format. The Humax is very unlikely ever to play a standard DVD (VOB format).