Thanks Richard, Good rnd.
I did the same exact thing for the first iPod I waterproofed at waterfi in 2011. I also experienced alot of leaking issues as did our early customers. Most of the grease only ones I had to replace for customers.
I eventually started mixing 1 part silicone adhesive and 2 parts grease (I think!) to create a jelly like substance when cured. I had used the permatex brand. This slightly congealed substance won't leak out. It will make the buttons slightly stiffer than just grease, but you won't have to keep filling it up.
You could just put the mix in a skinny syringe and inject it. Dont use a fat one of it will be impossible. Also you have to inject it quick before it currently or it will be too viscous to get in there.
But make sure you poke alittle hole in the bottom of the headphone port first with a paper clip. But don't push too far or you could hit the battery. Lastly clamp the iPod together lightly so the bottons and back doesn't pop off and really over fill it until it's oozing out everywhere. This will help insure you have flushed out all the air.
I think that's mostly everything for that method. I injected my first few thousand ipods that way and they survived pretty well. I think it's the best simple solution.
We're pretty well done with this process so I'm happy to help some diyers if I can.