The FDS
is a cartridge full of RAM that plugs into a Famicom. The disk drive part is only used to load data into that RAM, so aside from the loading part, it's just a cartridge. Yes, it is its own mapper, but a lot of its games don't use most of its features, especially if they were originally NROM games like Super Mario Bros..
The reason I'm suggesting it is that its mapper design has some similarities to the FME-7. It has a "fixed" ROM at $E000-FFFF which was used for its BIOS code, and for this reason an NROM game ported to FDS will have its data already relocated from there, usually to $6000-7FFF. If the game was only a 32k/16k PRG NROM, usually this will be loaded into the FDS once at startup and after that no other FDS mapper features will be needed, so at that point it's effectively a mapper very much like our proposed NROM-368.
Open the Super Mario Bros. FDS in an emulator and let it run until the game loads (e.g. get to the title screen), then do a memory dump from $6000-DFFF and you'll have all the data you need to build your PRG ROM. (Dump the 8k of CHR memory as well.) From there, just write a small startup program in the $E000-FFFF fixed bank that sets up your banks and nametable mirroring then jumps to Mario's code, and a vector table for NMI etc.
For an FDS game to be suitable for a trivial conversion to FME-7, it should meet the following criteria, which I believe SMB does:
1. Only loads data once. (If it loads data multiple times, you could write a replacement for the BIOS' loading code.)
2. Outside the initial data load, never writes to $6000-DFFF. (The FDS has RAM here so it's technically writable. You could use PRG-RAM at $6000-7FFF and load it manually for your FME-7 port. Ram writes at $8000-DFFF are non-portable, but likely extremely rare.)
3. Doesn't use the FDS' IRQ. (You could probably write a replacement with FME-7's IRQ but takes more work.)
4. Doesn't change nametable mirroring. (Requires replacing FDS mirror toggling code with FME-7 mirror toggling code.)
5. Doesn't use the FDS expansion audio. (We have a list of those games
here. This isn't much of a problem except that the extra audio won't play. I don't think the register writes are in conflict with the FME-7.)
6. The code in $6000-DFFF never calls BIOS code. (I.e. the code never jumps back to $E000-FFFF after the initial load. If it does, you'll need to write a replacement routine in the fixed bank.)
7. Doesn't write to CHR-RAM after load. (You could possibly run the FME-7 with CHR-RAM? Loading it then requires extra code.)
8. Doesn't write directly to nametable from load. (The FDS BIOS routines can load data from disk directly to the nametable. Requires extra code to duplicate this functionality.)
You can verify each of these things pretty easily with breakpoints in a debugging emulator like FCEUX. If all of the criteria are met, the port should to FME-7 be extremely easy. Even if they aren't met, they can each be overcome (except the missing audio, or possibly RAM use at $8000-DFFF) with a little work.