I have something large, but probably not as baller as an SAER pack. It's an ALICE that was heavily modified, reportedly by Tactical Tailor for a PJ. I don't know how true that is, but it's a solidly built pack. (If anybody has any additional info on this pack, I'd love some history) Note that the load-out shown is simply for perspective of scale. I load it specifically for whatever I'm doing, and keep it empty otherwise.
From what I can tell, it's got the 5 pocket modification, where pocket #2 and #4 are standard small zip pockets above the normal ALICE pocket. There's also a zippered opening in the bottom to access the main compartment, and a pouch under that that will take a compressed sleeping bag, blankets, tarps, or a roll up litter.
Pockets #1 and #5 are elongated and set to take O2 bottles. There're two retention straps, and pockets that will fit standard NRBs or Nasal canulas. I've also rigged a 3 liter camelback in one of these pockets, since there's no ready made hydration compatability
The #3 pocket is set up with three divided compartments (I added a galls pull bag to each to show size) and three organizer flaps. The large mesh side flap holds a large trauma dressing/burn sheet, and the two smaller mesh compartments fit 4x4 or 5x9 with room to spare.

This is the bottom compartment I mentioned earlier.
I do not know much about the SAER pack, but shy of trying to bring out an entire OR, there's nothing I could think to pack that I couldn't fit in this ALICE. Heck, a loaded STOMP II fits in the main compartment if you want to just go full blown silly. The only things left I would change would be to send it back to TT for a claymore pocket on the flap. If you look, there's a set of fastex buckles on the top, my guess would be for a smaller aid bag, which I don't have a need for, so I would prefer to use the space for further storage. I also would eventually like to upgrade the carry system, either with a NICE yolk, or an HSGI Ali pad, if I can find one.
My point being, for what you'll probably pay for an out of production mystery ranch pack, I'm betting you could get something built to your exact specifications.