From that RAM map linked a few posts ago:
0x001E Title Screen timer(time until attract mode) Counts down from FF to 00.
I'd wager this is what you're looking for. I don't have the skills to tell you what to do with it though.
I did verify that a cheat code of "001E:55" (I just chose a random number somewhere in the middle of the countdown, after watching that RAM Address' behavior for a while, and 55 was what I used) in FCEUX will completely eliminate the game entering the demo from the title screen, hitting Start still started the game as normal. You'd have to play the game for a while to see if this introduces any bugs.
If you can set your emulator up to load that cheat code with the ROM, then that may be your solution right there. A ROM hack solution would require more skills than I have.