Would a Link to the Past hack to make some sort of recreation of AST be too much of a hassle to make?
It really depends on how much you would like to keep. I'd say it's too much. For as much trouble as it would be, I'd almost think it would be easier just to hack AST to be one ROM and function more like ALTTP. Keep in mind though that either route would be an unbelievably massive and complicated work. They are in fact incredibly different.
Also, the game has 8 stone tablets. That ruins the easy option of just doing a graphics edit of the crystals or pendants.
What things are exclusive to AST that are not in ALTTP?
There were events that took place like rescuing Zelda and the Priest on the overworld while they are being chased/attacked by monsters. The majority of the events in the game relied on the timer.
There was that mole guy that would show up on the overworld in different places at different times. I forget what his purpose was. Maybe he gave you a heart container or something.
There's the rental shops where you can rent equipment like swords and maybe a shovel if I remember correctly. Interesting note, there is a cute graphic for the girl who runs the shop. She was not in ALTTP.
There are a few character graphics that are exclusive. Interesting note, the graphics for Sahasrahla in ALTTP only has a pose for facing forward but AST has at least poses for facing left/right.
AST automatically starts you out with a few items.
AST has a number of mini-games that ALTTP does not. I remember inquiring about transfering some of the mini-games over and I was told something to the effect of "the two games may look similar but they are coded very differently so no, it is not possible to insert the mini-game code from one into the other".
I'm sure there are more differences but those are the ones I can remember right off the top of my head.
Anyway, I hate to be a downer but using ALTTP to recreate AST just aint happening.