Well, strictly speaking, if you write code on a CD that will put "Hello World" into the Arcade Card somewhere, retrieve it from the AC and put it on the screen using the secondary graphics chip to do it...there you go, a demo that requires a SGX and an AC...even though its completely indistinguishable from a HuCARD demo.
Since the AC does nothing but store stuff, in order to really say "this game is an Arcade Card game" you'd need to make something TOO BIG for a SCD. That's not...too hard, as long as its a huge pile of sound or FMV. Its basically not possible to fill the AC with code unless you are running SETI@Home on it or some shit so what kind of graphics and sound fill the AC? The easiest thing, it seems to me, is video.
So is it possible to hack the HuVIDEO spec to the point where it won't run on a standard PCE but will run on a SGX? Like, maybe just adding 10 lines of resolution (ie: ten fewer lines of black box) and use the two GPUs in tandem to make it happen?
That would satisfy the SGX requirement, but streaming HuVIDEO doesn't use very much RAM so maybe cache a shitload of it and come up with something else for the CDROM2 to load.
Or...maybe overlays are possible? Could Road Prosecutor or Time Gal be make for SGX if one GPU did the HuVIDEO and the other did the overlays? If entire levels were loaded into the ACD to avoid (maybe) the lag that normal comes from seeking lasers in LD based games.
Just thinking out loud here...