Hey, Miles, couple of quick questions. First, what does the bulkhead between the back seat and the trunk look like? If it's solid, and the barrier between the tank and the back seat bottom is also solid, then you're good. If the trunk barrier isn't solid (ie has a cargo pass-through), then the mysterious metal discs you mentioned become fairly important. If they are fuel-tank access ports that can be popped off by hand, you'll probably want to replace them with bolted-on covers. If they're latched on in a way that won't come loose in an impact, you're probably OK.
ThePOS--I'm a little confused as to what's going where, but if the fuel system components that do intrude into the rear compartment ARE covered by solid bolt-on covers (it sounds like they are), that's probably OK.
In general, you're trying to cut off all possible routes for flames to travel from the tank area to the passenger compartment. The most common places we run into trouble are cars that have exposed trunk-mounted tanks AND some kind of cargo pass-thru via the rear seat, or hatchbacks that simply provide a clear path.