So how would I do this

Scripting!!! *gets shot*

In seriousness, you might be able to put triforce pieces in levels 9, 10 , 11, etc. and then write a small script to take the place of the "Level 9 Entrance Room". Basically, you would check to see if

if((Game->LItems[1] & LI_TRIFORCE == LI_TRIFORCE) &&
(Game->LItems[2] & LI_TRIFORCE == LI_TRIFORCE) &&
(Game->LItems[3] & LI_TRIFORCE == LI_TRIFORCE) &&
(Game->LItems[4] & LI_TRIFORCE == LI_TRIFORCE) &&
(Game->LItems[5] & LI_TRIFORCE == LI_TRIFORCE) &&
(Game->LItems[6] & LI_TRIFORCE == LI_TRIFORCE) &&
(Game->LItems[7] & LI_TRIFORCE == LI_TRIFORCE) &&
(Game->LItems[8] & LI_TRIFORCE == LI_TRIFORCE) &&
(Game->LItems[9] & LI_TRIFORCE == LI_TRIFORCE) &&
(Game->LItems[10] & LI_TRIFORCE == LI_TRIFORCE) &&
(Game->LItems[11] & LI_TRIFORCE == LI_TRIFORCE) &&
(Game->LItems[12] & LI_TRIFORCE == LI_TRIFORCE)) // etc.
{
	// code to change solidity/data/type of "invisible wall" combos
	// and/or
	// code to open the door(s) with Screen->Door[] (see build after 403)
}

Something like that. As for the subscreen, well, I just don't know.

Oh joy... what a pain.

Scripting isn't a pain. If you'd like some help, tell me what you need assistance with.