I'm here to report back on finding a replacement for Dunhill Flake:
First, I've changed my mind on Union Square. I can definitely distinguish it from DF blind. US is a different beast from DF, and while it's okay, I'm not, as it turns out, all that fond of it in its own right. (I was really into it for about a week, and then it just stopped seeming so great)
Second, C&D Opening night is fantastic. Certainly not the same flavor as DF, but a really excellent smoke and one I can (and do) keep going back to, bowl after bowl, day after day.
Third, OGS has grown on me a lot (I quit cigarettes about a month ago, and I think my taste is changing some--tobaccos that didn't used to do anything for me are now quite nice). Again, not a substitute, but like Opening Night, a great smoke that rarely disappoints. I am reaching for this one quite often.
Fourth, I just tried MacBaren H&H Pure Virginia. This one has a lot of promise as a replacement. Not spot on, but I think it is hitting many of the same notes. I'm still on my first bowl (and today is humid as hell, so not ideal for testing new tobaccos); once I've smoked a couple of ounces my opinion may change. But for now, this is the top contender for a substitute. (Maybe this will end up being another Union Square.)
I have tried a bunch of others, and there are some recommendations I haven't been able to get my hands on (anything by Fribourg & Treyer for example).