Well, I suppose this depends on whether you require colinearity of the continuation. oabyafK would not allow moves parallel to the edge fecause the f forces the test to be opposit to the move. But oabyaK would allow arbitrary direction change after the back-and-forth. Problem in the context of the ID is that it would generate each move 3 times, as there would be 3 ways for a King to step off-board and back (and in a corner even 5). You would really want the test to be a W move. (And even then corners are troublesome.) With a bracket notation that would be possible: [oW-bW-aQ]. I suppose you could achieve that also with vvssoabyaK.
But I still think uing morphing is the natural solution. You want a piece that has different moves depending on where it stands, and this was what the morphing was invented for.
Well, I suppose this depends on whether you require colinearity of the continuation. oabyafK would not allow moves parallel to the edge fecause the f forces the test to be opposit to the move. But oabyaK would allow arbitrary direction change after the back-and-forth. Problem in the context of the ID is that it would generate each move 3 times, as there would be 3 ways for a King to step off-board and back (and in a corner even 5). You would really want the test to be a W move. (And even then corners are troublesome.) With a bracket notation that would be possible: [oW-bW-aQ]. I suppose you could achieve that also with vvssoabyaK.
But I still think uing morphing is the natural solution. You want a piece that has different moves depending on where it stands, and this was what the morphing was invented for.