Slaanesh's Domination
"Action: Reveal up to three cards at random from one target opponent's hand. You may play any tactics thus revealed as though they were in your hand for no cost."

During his capital phase, the Chaos player plays Slaanesh's Domination. The following tactic is revealed:

Grudge Thrower Assault
"Play during combat, after damage has been assigned. Action: Destroy one target attacking unit."

The game is not in the combat phase but the Chaos player wants to play the tactic anyway (even if it grants him no effect). He intends to spend the tactic so his opponent can no longer use it. Is he allowed to do this ?

i'd say yes-

1) you pay cost of the card (zero in this case)

2) card is played & action is taken

3) card's effects resolve - oops! it's not during battlefield phase after damage has been assigned - have to discard card w/o effects resolving

much like a revealed/moved Hero in/to a zone with another Hero in it is my reasoning

You CANNOT play the Grudge. But now at least you know it's in his hand.

The timeframe for Slaneesh isn't exactly clear. It seems you may be able to play it at any time during you turn. It does say as though it were in your hand for no cost, but doesn't say anything about until the end of your turn, or do this immediately.

With an evergrowing pool of Tactics SD really needs a timeframe on how long you have to play the revealed cards.

A tactic is usually played, resolved and then discarded, so I would bet that if a ruling comes out for this, it will say the revealed tactics have to be played, and resolved during the phase you play it. Of course I could be wrong, and if you truly played those tactics as though the were in your hand, you could play them at any time including you next turn. IMHO, this card is poorly written.

You aren't wrong. Any tactic revealed must be played as part of the resolution of Slaanesh's Domination if it is going to be played at all. Once that effect is fully resolved the revealed tactics are unrevealed and you lose any chance to play them.

You may not play Grudge because it specifically says play during combat this is a play restriction which cannot be violated.