Description of the headers used in the SNES Classic ROM files (.sfrom)

snescheaders.md

Header

Start	Size	Description	Notes
0x00	0x4	0x00000100
0x04	0x4	File size	Includes the header and footer
0x08	0x4	Location of ROM	Always 0x00000030 in official files
0x0C	0x4	Location of PCM samples	Comes after end of ROM in official files
0x10	0x4	Location of PCM footer	Equals file size if PCM data missing
0x14	0x4	Location of footer (below)	Same as New 3DS's SNES VC header starting from 0x30
0x18	0x4	Location of S-DD1 data offset	Footer location + 0x2C
0x1C	0x4	0x00000000
0x20	0x4	Unknown	Footer location + 0x1B. Some kind of a flag?
0x24	0x8	Wii U Virtual Console Game ID	WUP-J**X, X — title region (E = USA, J = Japan, P = EUR)
0x2C	0x4	0x00000000

Footer

Start	Size	Description	Notes
0x00	0x1	Emulation speed in FPS	0x3C == 60
0x01	0x4	ROM Size
0x05	0x4	Size of PCM samples	0 if missing
0x09	0x4	Size of PCM footer	0 if missing
0x0D	0x2	Game preset ID	Varies between games
0x0F	0x1	# of players?	Usually 0x02. Setting to 0x0 disables the second controller
0x10	0x1	Sound volume
0x11	0x1	ROM Type	0x15 for HiROM and 0x14 for LoROM
0x12	0x1	Super-FX flag?	0x0C if the game uses SFX Chip, 0x00 otherwise
0x13	0x7	0x00000000000000
0x1A	0x4?	Unknown	Usually 0x00000100
0x1E	0x4?	Unknown	Always 0x00000100

I set 0x18 and 0x20 to 0x0 in the header and everything but ROM size and type in the footer to 0x0 for Super Mario All-Stars and it still works. It doesn't seem like the emulator cares about a lot of these values.

It looks like using this header with a combined footer (like the 3DS VC) works just fine for Super Mario All-Stars. If this format works for all ROMs, it would greatly simplify my conversion script.

      00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
0x00: 00 01 00 00 60 00 28 00 50 00 00 00 00 00 00 00
0x10: 00 00 00 00 30 00 00 00 00 00 00 00 00 00 00 00
0x20: 00 00 00 00 57 55 50 2D 4A 44 49 45 00 00 00 00
0x30: 00 00 00 28 00 00 00 00 00 00 00 00 00 00 00 00
0x40: 00 14 00 00 00 00 00 00 00 00 00 00 00 00 00 00

EDIT: Confirmed working with Final Fantasy Mystic Quest as well.
Mega Man X2 works with this header and a preset ID of 0x113D, except there's graphical corruption whenever the Cx4 chip is used.
The preset ID 0x1117 (MMX2 on Wii U VC) works fine.

That's interesting. I can't see any reason why it won't work since we directly point emulator to a footer.

So, preset IDs are incompatible? That's very disappointing. It looks like they rely on emulation profiles instead of emulating the SNES properly...
At least DSP-1 and SuperFX games work fine with a single preset ID (or do they?)

i think you should have a look at this:
https://www.reddit.com/r/miniSNES/comments/74z4oo/street_fighter_alpha_2_running_on_canoe/?sort=new