Well there is a plugin called Score Rewards that kind of does what you are describing. I've seen it used elsewhere to good effect. Plus the invisbility reward allows you to go around surprising people by stabbing them in the face... so much fun.
// REWARDS:
// ========
//
// 1.) Health refillment
// 2.) X ammo for weapon currently in hand
// 3.) FG42 with 999shots (any map, any team)
// 4.) X seconds godmode while glowing in team's color
// 5.) X ammo, +X kills, -X deaths
// 6.) X seconds invisibile/silent
http://www.dodplugins.net/forums/showthread.php?t=39