You CAN play as Fierce Deity Link anywhere in the game!
By: Cameron Laventure
   

I'd just noticed an article "is the Fierce Deity Mask worth it?" Well I've
found that you CAN play as Fierce Deity Link anywhere!
All you have to do is go to Sakon's lair and wear a mask as Link. As Kafei,
replace it with Oni Link Mask. When it's Link's turn, he'll become Oni
Link. However, he cannot take the mask off, so nothing must be worth
saving. Complete the puzzle. You have until the moon falls, but I must
give you some warning:
Do not talk to a Bomber kid or game will freeze
Do not go into Stock Pot Inn as you can't get out
You cannot use any items except for the bottle, I know, strange
Once in Clock Town, you can't get out
Okay. You can get out of Ikana Canyon using the river to get to Southern
Swamp.