Semi transparent camera in free unity

I have 2 cameras in my game, a main one in which the player sees the game and a second one acting as a minimap. The minimap can be dragged to anywhere on the screen and resized to suit the player, however I would like to add an option to have the minimap semi transparent (adjust its alpha) so that the player can see the main game behind it.

I did find one way of doing this using render texture, but it required unity pro which I currently can not afford. Is there a way to do this in the free version of unity?

In case it makes a difference, the game will be published using the webplayer.

No, this can’t be done with a camera setting. Store a collection of all the materials used and alter all of them in a foreach loop.