I have to assume the thread about the "Japanese drift comp" at Tsukuba with the funky rules inspired you to make this thread? Anyways, I'll add my two cents.
There's nothing wrong with it, as long as they are properly organized, rules are clearly explained, and a level playing field is established (in terms of car restrictions and regulations).
I'm not quite sure exactly what you mean by "out of the norm"...do you mean different tire compounds? Different drive trains? Or something else different all together?
I honestly think the number one reason we don't see FF, AWD, or various tire compound comps is simply a lack of interest.
I don't have anything against FF or AWD drifting, it's hella fun from time to time...and if someone prefers to drift on tires other than CH, that's cool too. But personally (and I think a large portion of the community will agree with me), I simply don't have the interest to tune a FF or AWD car to the point where I feel it is comp ready (same can be said for tire compounds, with CM being the exception).
Another reason I think people avoid FF and AWD drift comps is the judging. Do you have any idea how to judge a FF tandem run? I sure don't. I, along with pretty much everyone else who plays GT5, have limited time that I can play the game. I have no interest in joining a comp where I foresee any kind of major debate over judging occurring, as that is a waste of my time. I would rather drift for fun with friends in my limited play time as opposed to sitting around arguin about comp rules.
As others have already said, this topic has been discussed at great length on these forums. People's biases aside, the general consensus is that different drive trains just don't drift well with each other due to the difference in the physics (which leads to the problem areas, those being line and speed).
I agree in part with your second post in this thread, where you say it's possible for a RWD and AWD car to drift for fun together...but I disagree if you think they can fairly compete against one another. It's important to remember that there is a difference between tandeming for fun, and tandeming in a comp.
To me, asking why RWD and AWD can't compete against each other is like asking why skateboarders and BMX riders don't compete against each other in the half-pipe at X-games (snowboarders and skiers might be a better example). Putting the fundamental basics aside, the two disciplines are quite different from one another, and just don't mix. You can fairly race one against the other (RWD vs AWD, or skier vs snowboarder), but you cannot judge one against the other in a fair way.
I'm going to play Devil's advocate on this one. You make it seem as though the current "unwritten comp rules/standards/regulations" have been set in stone for the last three years. The other way of looking at it, is that it has taken the community 3 years, of trial and error, along with a fair amount of discussion, arguing, bickering, fist fights, and death threats, to get to the point we're at.
Using that "Japanese Drift Comp" as an example of an integrated drift comp...try to imagine how that would have gone. Do you really think someone would show up in a FR car, with 400ish HP, on CH tires, and then try to compete against an AWD car with 700+ HP on CS tires? I highly doubt it. If that comp would have actually gotten off the ground, I'd bet almost everyone in it would be running Skylines on CS tires....and how "integrated" is that?
Putting the drive train and tire compound issues aside, that Japanese comp didn't stand a chance of going anywhere due to the alarming red flags some of those rules were putting out (1 corner; 400hp minimum

; DS3 only...wtf; and most alarming, whatever he said about front tires not being allowed to drift, or whatever it was). All of those rules clearly demonstrate a lack of general knowledge about drifting...which makes interest nose-dive.
I highly doubt you will ever see comps where different drive trains or different tire compounds compete against each other, simply because it is like judging apples vs oranges.
I think someone (perhaps you could be the one to step up to the plate Rob

) could potentially organize a successful AWD only, or FF only drift comp. The biggest issue I foresee would be finding enough people interested in joining.
I think one way to add some variety to the current line-up of comps is to have more themed comps (like Marbel's JDM comp in July, or Slick's D1-4-Fun 4-Door comp a few weeks ago). There is all kinds of potential for a wide variety of themed comps. One theme I'm actually surprised I haven't seen yet is a one make comp...ie Spirit R only, or Miata only, etc etc.
Anyways, those are my answers, sorry for getting long winded...I'm bored at work lol.