Okay, update. I've found that there are TWO addresses in memory that need to be at specific values. first of all, you were correct in assuming that we would need to make the game think we're above ground. BUT we also need to trick the game into thinking we're on world four or higher. so, now that i have the specific variables needed, ill look into possible manipulation tactics.

I've looked in to this a bit and sadly it doesn't seem possible. where the warps take you is stored as a byte which is seemingly unaffected by anything other than your x value in relation to the current level. I'll look into it more and let you know what i find, but it isn't looking good at the moment.