I have in the past done valve collets on a cb500 by pressing hard on the tops of the collett retainers with the open jaws of a adjustable spanner, in my own circumstances I was from lack of the right tools and the head was laid flat on a soft wooden bench. I have used the string down the plug or injector hole to stop engines turning over a fair few times, so I cannot see why the 2 should not be combined, I think I can go as far as saying that I have even used the string trick on a sohc Honda, though not one of the 4's.