... man,,,, I feel your pain,,,,, those little bastard pinholes are hard to find some times!
What I think is happening is the pinhole will only leak enough for you to see when its under normal pressure, eg 6 psi. It probably won't show easily when the bladder is out side of the kite because you can only safely put, say 1/2 or 1/4 psi in, which probably won't be enough to see the bubble. The little bubble might only pop out one small bubble every 20 seconds and by then, you've moved on looking else where. If you try to pump the bladder up, to any decent pressure, to speed up the bubbles, outside of the kite, the bladder will burst.
What I've done is exactly what you've done so far but this time, with bladder in the kite of course, lather the LE with dishwashing liquid, watching exactly where the liquid bubbles show up, taking careful note the actual measurements where they show up. Use landmarks to measure the bubbles position i.e. use the closest valve is a good one. Also, take note the pinholes orientation compared to the valve too, for example, it may be 90 degrees around to the front, relative to the valve.
With measurements written down, take the bladder out and mark the bladder so you now have a general area of where to look. Go to pool and pump the bladder, enough that you don't damage it of course and hopefully will still see the air bubbles.
This is the hard bit, the bubble can be super small and very slow to bubble out so watch and wait, be patient not to move on too quickly, of course starting with the area you've marked. Slow and steady is the key. If you suspect that you saw something, not quite sure, stop and stare at the area,,,, and wait for it to happen again. You may have to wait some time, be patient, but when you see the f'ing bubble, keep staring at it and bring the bladder to the surface, grab a pen and circle it while keeping your eye on the bubble the whole time. Your nearly there.
Once you identify it and circled it, double check your mark and put the bladder in the pool again and wait for a new air bubble to show. Hopefully you will see it pop out of your circle. If yes, you found it. If no then circle it again with your pen and now triple check it.
If you see the tiny air bubble again, inside your the circle mark, bingo, dry the thing and patch it.
Yay, your done. Plonk it back in the kite and hopefully you don't find the guard tower again.
I hope this helps.