Coach Rippetoe: Apologies for the original post in QA. I did search the forums before my post, initially with little success. Having refined my search somewhat, I did manage to get the answers I was looking for.
As a result, I'm largely going to reply to this (my own) thread, having searched the forums more thoroughly for an answer.
1. Non-round plates suck, this is not a question that needs asking. It's something everyone agrees on.
2. Unless you can encourage your gym to change plates, have them buy full-size, round spacer plates, or take along your own, you are shit out of luck.
3. If you can find just two, round 45s amongst the sea of shitty plates in your gym, you can stack these on first and negate the impact of the remaining hex/oct/dec weights.
Otherwise, yes, you are basically stuck with a sub-par environment for your DL, and you will have to reposition as well as de-weight between reps.
As an aside, while searching for answers, I did learn that you can use the hex plates with the grip holes for penis exercises. Just shove your weiner through one of the grip holes and rep away. Ingenious.