I did have an idea the other day and today thought I'd try it. Although I'd made a mistake in my code and it did not work (plus something else, namely I think needing to shuffle a bit of code about) , the idea I thought may of had merit.
Two ways of doing it, either set the airbourne counter to #FF and jump back into the main loop or drop into the collision detection "kill" routine. It does seem slightly confusing as to if a POP is needed here to zap the last return address off the stack however.
Simply put the idea was : The Portal is fatal on contact unless its active.
The Kong caverns might be possible if jump and left is held down (not tested!) as M.W will not be in contact with it if he walks left under the earth wall with jump held down, causing him to leap up as soon as possible.
Just a passing thought...