[dbbubba]

The chord progress was defined by the OP as:

A / mystery chord / D / A

Unless those four chords are a fragment of something else you can't really question whether the root key is A. Standing alone, that progression is in the key of A.

If you were writing this chord progression it out in notation it would be in the key of A.
If it was a fragment of a phrase in another key it would require A LOT of accidentals.

I don't think that we are dealing with a chord progression or melodic situation that is THAT complicated.

BTW... the "rules" are written to make stuff sound "right" and like we expect music to sound.
If you want to break those rules you had better know WHY you are doing it.
Otherwise it is vague at best.
In the late 1800s composers started to "break the rules" but there was a logic and reason behind it.
It wasn't out of ignorance or lack of understanding "the rules."
In fact, almost all of the guys who broke the rules started out writing music that rather strictly adhered to the rules.
The beauty in knowing how to follow the rules and also how to creatively break the rules is that you can do both.
When the guy who writes "modern music" gets the call to write a John Williams type movie score he can actually do it.

KAFKA has the Frank Zappa quote from "Joe's Garage" at the bottom of his posts and that brings up an interesting point.
Although there is no doubt that FZ was a musical genius in every sense I bet that it would have required A LOT of thougt and effort for him to write a complete John Williams type "Indiana Jones" type score.
He could do it, but it wouldn't come as easily as writing the type of music he (FZ) wrote.
FZ learned by bending the rules and that's OK (I am one of the biggest FZ fans there is.)
Frank would have avoided the hassle by saying "Phhhheeeyyyyooooooo!!!!! I HATE that type of music!"
Could John Williams write FZ style stuff... no, but the market for his style of music is FAR greater.

If you are going to bend the rules and make non-traditional music you had better have a good day job, be prepared to starve or be a genius at marketing your music.... or all three. FZ did all three of those at points.

I have seen lot's of metal GTR players doing this stuff.
With the heavy distortion and the overtones it produces there is often an interesting sound produced.
Still, broken down it is "wrong" and often requires the overtones produced by distortion to sound "musical."