Hey Bob,
I went to Cocos in June last year and it was without doubt the best diving I have ever done. I am very jealous of you.
It was my last trip with a film camera (MMII) before upgrading to digital. I have to say that my pictures were terrible. This is for a few reasons which make photography difficult there.
1) Overcast sky - the sky was overcast almost all the time we were there, leading to dark conditions underwater
2) Thermoclines - Expect to encounter these quite a lot. A real pain if trying to shoot through the shimmering.
3) Pesky Hammerheads - They really do hate the bubbles. Getting close (I was using a 16mm lens) is really hard so you have to take your chances. A rebreather really does help.
4) Currents - Some of the best photo oppurtunties were in currents so strong that you couldn't even look into the current without losing your mask. Take a pair of gloves or a reefhook (there is no coral to damage)
5) visibility - not always the best. A lot of hammer shots you can't get close enough so just turn off the strobes to avoid backscatter.
I am sure you will make a better job of it than me. 
As regards panning - the hammerheads don't tend to move that fast so it shouldn't be too tricky. Now the blacktips and silkies can be a different matter.
I found one of the best shots (at Alycone) was to lie on your back and shoot up with the Hammers schooling above you for good silhouettes.
Anyway hope you have a great time.
Mark