I have something, which may be useful to you or would be able to give you some ideas. In my ghosts plugin when a player dies, it turns into a ghost. Not much to interact with, or to harm others, just to show, where is a dead player.
Making spectator team - or dead team - playable would be a real big job. Rehooking all the buttons, give model to the player, weapons and alike. ( Though I am interested, tell me if you need help.

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I would much rather make use of dead players in cstrike. If a noob goes to a server, he dies much, he spends his gaming time mostly dead. So this would be amusing, because death would not mean the end of the game until new round.