Only if you put it facedown on the battlefield. You could make it work with a text like:
Bluff N (Pay , where X is N or higher: exile this card from your hand facedown. You may turn it face up and cast it without paying its mana cost whenever you could play an instant.)

That doesn't work either, for the same reason you can only use the miracle cost when you draw the card.

Okay, how about attaching it to creatures?

Bluff - When [cardname] enters the battlefield, you may exile a card from your hand face down.
When [condition], you may reveal the exiled card and cast it without playing its mana cost if able.

Then have the condition be things like the creature being dealt damage, or targeted by a spell or ability an opponent controls.

Basically the term you're looking for is "Trap Card," as in "You've activated my ~"

That last iteration is potentially interesting though.

Without any specifics, I can't provide any solid comment on how it works with the rules. However, there are many potential issues outside of face-down cards being defined as 2/2 creatures, which is fairly easily changed.

Edit: CKY had a spellbluff mechanic, but I wasn't entirely thrilled by it.

I wish CKY had come with us to NGA... I like her stuff.

We must lure him here. If CKY is still on WOTC pe5rhaps he can be convinced...

It's about to happen. Next card posted decides the set that the next pack of cards are created for.

Hah! I'm not the only one who assumed he was a girl!

I started it. If you have a better description of Jakkard than the one I copied off of this thread, then go ahead and post it.