As nice as that sounds, the end product would be expensive and huge (the Express itself is expensive and huge). People would still probably never actually leave the house with it. HuGoPSP (or something like it) is still going to be 1000x more practical since it does all that, and plays CD games (theoretically, I've never had it work).
Oh, I know. I realize this. Hence why I asked. Emulation isn't real hardware, and some people prefer the real deal no matter what. This would be specifically for them.
None of the flash cards for the PCE that I know of, emulation the back memory hardware (the lock/unlock register and mapped save ram). I think the main reason they don't, is that you would get bus conflicts if you attempted to access that feature on the card - plugged into a core system with a booster or a CD addon/system. That's the last thing you want when trying to update something as important as BRAM (for both the card and the conflicting device on the system). I imagine you could put a switch on there, but there will always be someone who wouldn't understand or pay attention... and goodbye BRAM save data on your booster/CD system. Matter of fact, the Tototek card has problems with the SuperCDROM^2 addon. If you use a hucard game on the card that tries to access BRAM in that setup, you get corruption. Doesn't happen on the Duo. Matter of fact, trying to access anything in the upper 8megabit region with the tototek card in that setup - gives you problems (64k CDRAM, even had problems accessing hardware bank mapped ADPCM registers). I haven't heard of any problems with the Neo Flash, the SuperCDROM^2 addon, and accessing BRAM.
The cost to build this BRAM emulator and pass through, would be about the same price it would be to make a flash rom card. You wouldn't have the issue of the pass through connector (and its cost), but you would have the added cost of the additional logic/flash rom. The big difference though, is that the flash card could be open source. And possibly have the ability of being able to make it yourself, if you're up to such a DIY task.