At the end of the day, an 8Mb Alice would only be able to see 8Mb in an A4000, since its the only Amiga motherboard in existance with enough address pins wired to the Alice chip.
Leirbag, just face it. The only things that would get you 8Mb of chip ram in any way compatible to existing amiga os & apps are:
1) Run UAE
2) Make an 8Mb Alice (and possibly revised other chips), and put them in an A4000
3) Emulate the whole chipset on the 68k (which is basically 1)
And thats it. All of the "so-called" impossible things you mentioned (memory cards, USB etc.) were feasible if someone came up with the hardware and drivers to do it. 8Mb chip ram is something different.
With appropriate glue hardware you can interface pretty much any peripheral to an Amiga (or C64, or anything with some IO ports). You could add a firewire port to a ZX81, but it wouldn't actually be useful ;-) ). But 8Mb chip ram just isn't possible in the way you want it to be done. It just isn't.
In fact, i'll say this. If you don't want to run AmigaOS, or any Amiga apps on it, you can add terabytes of chip ram to an Amiga. Its easy, you just put a PCB on the motherboard over the chip ram chips that has address decode logic that includes banking. Lets say, for example, writing a number between 0 and 4294967295 to address $C banks in a different 1Mb bank into the upper 1Mb of chip ram address space, and you actually had that much RAM on the board. Thats more than 4000 terabytes of chip ram.
Great. Except that AmigaOS and its apps simply would not be able to use it. You'd have to patch amiga os, and pretty much rewrite all of your software. It just wouldn't work. So get over it.
Edit: And before you say "Great! We've just got to fix the problem with software compatibility and its solved!", don't. Fixing it is more work than it gains you.
You'd have to change exec to remember the upper memory bank number for every task. That parts easy and has almost no speed penalty.
The problem is that AmigaOS and its apps expect to be able to pass memory pointers to each other. Now, you could pass 64bit pointers instead of 32 (32bit address, 32bit chip bank), but now you have to patch _every_ single Amiga application individually that you want to run, because every single amiga app and API uses 32 bit addresses, and you wouldn't be able to run a single existing binary natively without modification.
