And here a reply to the actual question ;-)
The reason for the 128 MB maximum on A3000/A4000 accelerator is, that that is exactly the adressspace that was reserved for the fast-slot by the engeneers of commodore.
In theory you could do something similar to the addmem tools (in hardware) to access RAM in an adressspace outside of that window. P5 decided against that, as that would be a hack and they didn`t want to do this kind of thing, plus back then 128 MB was more then enough.
The A1200 does not have this problem because they decided to use that kind of hack there though, as the designated adresspace for the A1200 trapdoorslot is only 8 MB and that was way to little.
I`m not sure if the 256 MB limit is more a matter of 128 MB modules being the bigges available or if the memmorycontroller used only can adress that much RAM.
To make it short: No there is no way to make an A3000/A4000 accelerator take more then 128 MB unless you are willing to redesign the whole memmorysubsystem of the card.
