In my experience, you can run an all Ork pretty well game by using the rules in RT with a few more modifications to make the different Orks feel more diverse, mostly by butchering the DW rules. Most important is a rework of the clan/oddboy rules into something a lot more meaty, akin to what you would have in Deathwatch (Deffwotch for ya gitz out tar'), with the basic Freebooterz class acting as a set of generic advances any Ork who makes a living barreling through space in a hunk of steel held together by chicken wire and a few bolts ought to be able to take, Da Klanz acting as Chapters, and the Oddboy choice as an actual Specialty. You can even mod/port the Solo and Squad mode abilities, as they fit the Ork, both their extreme adaptability and their improved performance caused by the strange psychic interaction between groups of them. Just rename Cohesion to WAAAAGH!!!, and you're set, you can even decide who's Da Boss for different missions/objectives based on the individual Orks' strenghts. But da Kaptin' stays da Kaptin', unless someone gets a bigger hat. Which is unlikely, since da Kaptin' has the biggest, baddest, dakka-est hat in the galaxy (needless to say, this last part is mostly a formality, ala Rogue Trader class in RT, since Orks are even less respectful of proper authority than you're average RT character unless another Ork is quite a bit bigger than them and/or has a much more impressive hat if you are playing Freebooterz).
All in all, it works well if the GM and players are knowledgeable in older Ork fluff, especially 2e ed clan stuff and Gorkamorka (and, if you really want to have some fun, 1e ed stuff like the old Madboyz rules), as you can really ramp up the
silliness orkiness without losing anyone at the table. Also, as a GM for such a game, don't be afraid to make stuff up on the fly, and to let the players do so, as long as everyone is having fun. For Mork's sake, you're playing Orks, creatures who can change the physical properties of objects they own based on their color; if the players get a cunningly brutal plan, or even a brutally cunning one, let them do it, and maybe throw in a bit of Paranoia-esque consequences to add more fun.