the problem is 32K of SRAM is nothing. even 256K of SRAM is nothing. the FPGA has plenty of blockrams on it in this size or better, and can run much faster- 150-200MHz or more. The problem is bulk storage (i.e. neogeo carts) and sometimes raw speed (tg16). Many tg16 games won't fit into 8 32K SRAMs. The problem with SDRAM/DDR is latency. These systems require random access to the storage medium to run the game, and SDRAM/DDR is designed for burst access. i.e. you precharge, open the row, and burst out 2 or more words of data really fast.
With older videogame stuff that had asynchronous memory, they expect to present an address and read data X nanoseconds later. Shoehorning this into SDRAM access means the SDRAM has to run through the entire access process for each byte/word the system expects. i.e. precharge, open the row, send address, read. each of those things take a certain number of clocks to perform. There's also refreshing to worry about. The refreshing can be spaced out using "hidden refresh" but this also increases the latency as well.
The tg16 wants data at 7.2MHz which is pretty damn fast when it comes to random access on the SDRAM, even when the SDRAM is running at 165MHz. For PC style emulation this isn't such a huge problem- they can take advantage of bursting and caching to get around a lot of the problems with FPGA style low level hardware emulation.
Flahscart works today, surely you could do the same (the NeoSD is not super fast during first flashing but it works like 3m:30s to load KOF2003, Turbo EDv2 uses RAM and it's very very fast).
I thought the issue you were facing was the actual RAM the system had and not the storage for the ROM, an all RAM solution would be awesome if possible but if not then fall back to flash would be perfectly acceptable. The total size of a NeoGeo game is <= 128MB, the PCE has the biggest mapper game at 2MB (SF II) and as you are aware all games run today on NeoSD and on TurboED v2 (which has 4MB RAM onboard) .....
I'm not questioning how hard/easy just stating it's been done for PCE and flash would be an acceptable fallback for NeoGeo if 128MB of SRAM or Pseudo/1T SRAM is too expensive.
Just brainstorming here, not attempting to derail your line of thought/project
is this availble for cheap?
you'd need 8 of them for 128MB assuming you don't need to support weird splits.