Endless possibilities - but everyone thinking along the same route?

[shonky]

Also, metal music is heavily atonal?

Not really, if an electic guitar is playing a C chord heavily distorted, it's still a C chord plus some noise. It could be either way. Somebody should have given Schoenberg an electric guitar first, I guess.

Reading this immediately made me think of Slayer's Angel of Death for some reason. Ostensibly, you could say the tonal centre is E (or D# due to detuning) as the riffs tend to feature this note as a constant pedal note but there's very little historically conventional in how the other chords relate to that note (admittedly, dissonance, predominance of diminished fifth and semitone progressions in thrash, death, and black metal have become pretty cliched since AoD was recorded). The solo is certainly atonal, seems to have very little relation to any key, often tending to go for the most dissonant notes without much regard for the underlying harmony - still unsure whether he's a really out-there player with some free jazz/experimental noise leanings or just not that good, although to be honest the sound fits the tunes' feeling of chaos.

Any song that employs the diminished scale is technically atonal.

How that does happen?

Similar to how it works with the wholetone scale, due to the way the scale is constructed, the tonic could be any one of four chords (6 in the wholetone). Essentially if we say:

C Dim = C, D, Eb, F, Gb, Ab, A, B, C

then we can see that following the same scale division that Eb Dim, Gb Dim and A Dim all contain the same notes, hence unless the underlying melody and harmony indicate one of these notes as a tonal centre that it could be any one of these four (it also doesn't help that the dominant and subdominant are also the flatted fifth and fourth respectively).

[jancivil]

A diminished chord as a harmony on a tonal center doesn't work, though.

One of the attractions of the so-called diminished scale is to take you outside. That said, a typical use in jazz is to grant a dominant V-type chord full extension, explicated or just implicit in the line.
Take the second version of what you spelled: D Eb F F# G# A B C, D7b9/#9 #11 (or b5, as Ab) 13. But back to my first point, the diminished triads embedded in it create multiple key areas as possibility.

And, that said I heard Leningrad Cowboys guitarist toss it in in Ring of Fire where it just comes across as deranged for a quick moment.

[shonky]

A diminished chord as a harmony on a tonal center doesn't work, though.

But back to my first point, the diminished triads embedded in it create multiple key areas as possibility.

I think this was what I was trying to explain, albeit rather clumsily. I would say though that some music can appear to set a tonal centre through repetition of a riff, drone, melody or pedal tone - if we looked at the intro riff in Black Sabbath by Black Sabbath for example, I think most people would hear that centre as E dim with the fifth as passing note trill rather than Bb dim (admittedly, it is much easier to establish a centre when you're only using three notes).

And, that said I heard Leningrad Cowboys guitarist toss it in in Ring of Fire where it just comes across as deranged for a quick moment.

I think one of the oddest places I noticed a diminished arpeggio was in the Rufige Kru's Terminator, although I'm not sure they necessarily realised it at the time. It's the James Brown Funky Drummer beat played in ascending minor thirds using the revolutionary new (at the time) timestretch facility in samplers.

[jancivil]

tonal centre through repetition of a riff, drone, melody or pedal tone - if we looked at the intro riff in Black Sabbath by Black Sabbath for example, I think most people would hear that centre as E dim with the fifth as passing note trill rather than Bb dim (admittedly, it is much easier to establish a centre when you're only using three notes).

I don't, I don't think that harmony is there. Been ~47 yrs, but I remember the single notes E, E an octave up, Bb.
Seems like power chords but not certain. Even if not explicit, the timbre of that much overdrive gives you 3rd harmonic (8ve + P5) anyway. So if someone played a diminished triad, it will really clash.

the Ring of Fire bit was the octatonic scale.
once and then back to 'normal'.

[shonky]

Well, diminished no third (or just a tritone basically), played as a ridiculously simple arpeggio, but do you hear a tune in E with a flattened fifth or a tune in Bb with a sharpened fourth?