I was wanting to do this also and have seen wires clamped together (as in pic above) or even wire tied together without using wire seperators. It makes the clearence issues a little better when they are bundled like that, but will this not eventually cause a problem with the wires arcing across each other??

I have run my wires under the headers for over a year now and have not burned a set of wires yet. When I was running them over the heads I went through 2 sets in less than a year. I am using the thundervolt wires(10mm). I have them zip tied together and some of them pulled up against the block to keep them off the headers. I used this set up on my 468 and now on my ZZ502 with no problems