There are no holes other that small opening on where air line use to go through which I sealed with black silicone. #27 here:
RealOEM.com - Online BMW Parts Catalog
Look at the pictures in post #19 of my link and you'll see that all is sealed.
The small plastic cap I mentioned is also visible in one of the pictures but its main purpose is to set coils spring in place.

That's not how BC's install. On BC Coilovers the rear spring is on the shock, not a separate piece like in your setup.

I vaguely remember my non air ride car had a metal plate blocking off the hole for where the air spring sits. I can't find it on any of the diagrams but there should be something out there for you to use.

If you originally had non air ride than your setup was this:
RealOEM.com - Online BMW Parts Catalog
#'s 13, 14 and 15 along with coil spring instead of air spring.

Correct, I forgot.
Anyway, just seal that hole with black silicone and you're done.

Right, but I believe there was some sort of metal cover above 14. 14 is just a piece of plastic/rubber that was held in place by 15.

Stumbled upon this thread while researching an issue I'm having...

I have BC's installed on my 4.6 and at speed (If I recall correctly above 50mph) it acts as if I am towing a VERY imbalanced trailer, it will violently sway side to side if I'm not careful.

I left the rear bags in place but I disconnected and emptied them... any ideas what the cause could be? I never had these symptoms before, except once or twice when the rear bags had drained and failed to air back up, it would sway when the rear was squatting... at normal ride height it drove great.

I know rear control arms can cause this, but it feels much more violent than when my other cars have had bad RCAB's. I thought it was related to the fact I drained the bags completely, but it seems the rest of you have done the same or removed them entirely. Thoughts?

Edit: After more thinking, I haven't yet got an alignment, perhaps there's a relation between the lowered ride height and unstable handling. Think the toe could have changed enough in the rear as a result of the drop?

You might be mistaken. Remember, X5 wasn't intended from factory to have coilovers in back and therefore they wouldn't produce part number to cover the hole which was never intended to be empty.

I'm not engineer but your case solidifies my decision vs coilovers. My problem with them is that X5 rear suspension was designed to have 2 points of body contact (shock and coils spring / air bag).
Coilovers effectively change that into one contact.
Your issue is probably combination of several factors and it may not have to anything with coilovers. As I said, just my best guess.

Mine is completely stable. It's likely from worn bushings, a poor alignment, an issue with the tire, or any combination.