I stumbled across an Apple KB article that mentions the software I was thinking of: "Connectix Maxima". I gather from related googling that it didn't actually relocate the ROM, it stayed at the 8MB mark, but, depending on how many Nubus cards you had, gave you a second block of up to 6MB of RAM, with the proviso that a single program couldn't use more than 8MB-minus-system-overhead. (You could, for instance, load Photoshop or something into the bottom 8MB with a maximum allocation and then use the multifinder to load some smaller programs into the extended space you'd carved out.)
The fact that this exists does suggest that mapping another, say, 4MB onto a PDS card and using it to run MOAR programs might just be feasable. (Since unlike the compact Macs the Portable actually crams its I/O into a compact space, instead of smearing it all over the top half of the address bus.) You'll need something like a patched version of Maxima to do it, and I'd still say there's likely to be formidable problems with going over the 16MB mark for anything but a RAMdisk. (Unless miraculously the Portable's ROMs are 32-bit clean, as would have to be any software you'd like to run.) If you're *really* dreaming big I suppose you could talk about going with a 68030 (you'll need the MMU) and soft-loading a custom Mac II-caliber ROM, essentially converting the Portable into a PB140-class machine when the accelerator is enabled. I'm sure grafting the drivers from the Portable's ROM into a 32 bit clean ROM image is something BBraun could do over a long weekend. Go for it!
The fact that this exists does suggest that mapping another, say, 4MB onto a PDS card and using it to run MOAR programs might just be feasable. (Since unlike the compact Macs the Portable actually crams its I/O into a compact space, instead of smearing it all over the top half of the address bus.) You'll need something like a patched version of Maxima to do it, and I'd still say there's likely to be formidable problems with going over the 16MB mark for anything but a RAMdisk. (Unless miraculously the Portable's ROMs are 32-bit clean, as would have to be any software you'd like to run.) If you're *really* dreaming big I suppose you could talk about going with a 68030 (you'll need the MMU) and soft-loading a custom Mac II-caliber ROM, essentially converting the Portable into a PB140-class machine when the accelerator is enabled. I'm sure grafting the drivers from the Portable's ROM into a 32 bit clean ROM image is something BBraun could do over a long weekend. Go for it!