I believe this taunt should be treated as a partner taunt (such as the High Five, RPS, Square Dance, etc.), where the spy will stand there, offering a game of Russian Roulette to those passing by.
If somebody decides to join in, the spy will take the first "shot" (either a missing round or the actual bullet). If the spy doesn't have the bullet, he will hand the revolver to the other player, who takes it and does the same. Once a player dies...
-If the spy (who offered the taunt) dies, it will just be a normal death.
-If the other player dies, the spy (who offered the taunt) will crouch down and pick up the revolver, which will drop near the dead player.
Then, whoever wins can walk away, while the dead player will either have normal respawn time or maybe have a modified, instant/shortened respawn time.

On another note, if this were to be incorperated into the game, they could add new voice clips for each character. For example, the spy (while idling) could say things such as, "A game of Roulette, anybody?" "Don't be a coward," "Who has a death wish?" and so on. Then the two players could take their turns, each making a relieved sound (e.g. a sigh of relief) if they weren't shot. If the spy (who offered the taunt) died, then the other player could walk away silently, maybe freak out, say something like "what a shame," etc. If the other player died...
-And was on the spy's team, the spy could mutter something while picking up the revolver such as, "Rest in peace, amico," "You were a brave man," "Farewell," etc.
-And was on the enemy team, the spy could mutter something such as, "See you in hell," "Good ridance," or maybe even his already existing line, "Well, the moment has passed. Back to work!"

So, everybody, feedback?