From what I have seen, you are going to have to go that extra mile and snag at least one defensive trinket until you get access to better gear. The enchants will also help you get closer to your goal. Remember, you only need 535 to be uncrittable for heroics, versus 540 for raids. It might not seem like much, but that extra 5 defense can allow you to swap one piece of gear for more avoidance or stamina/damage.

As a pally tank, I have only run with dps DK's so far. I would plan on DPSing my way to 80, while grabbing tanking gear along the way. The DK in my guild that wanted to tank used this method with much success. I would advise not rolling on tanking gear in an instance unless your current tank passes on it. This tends to piss off the tanks.

It is nearly impossible for a DK to get to 540 defense without heroic gear unless you have the Seal of the Pantheon from Loken in regular Halls of Lightning. Hopefully, you'll be luckier than I was, as I had to farm him for nearly a week, doing at least two runs a day before it finally dropped. Hence why they're coming out with the +25 Def Skill runeforge for the next patch... to make getting a DK uncrittable less of a Herculean feat.

That said, running heroics as DPS is a reasonable way to get some gear (although DPS can easily be undergeared as well... make sure you can sustain at least, say 1100-1200 DPS to keep your group happy). I don't think that deferring to the tank, especially in a PUG, is necessary, but stating your intentions when you join the group should be common courtesy. Expect to be asked to leave a small percentage of times when doing this... hey, the guy who's tanking might have been farming this item for days... but most of the time they'll say fine. Even better, find a Bear tank :P