A similar question was posted here a few days ago. Some of the suggestions there include:

• using guiText/guiTextures
• switching your text fields to use the GUI.Label style (ie non-interactive)
• catching input before calling the GUI

Hopefully it will be useful to you.

Found a nice solution to this, looks like the behavior is not to hardcode alpha to .5, but to multiply it by .5. Therefore, setting the color to (1,1,1,2) results in the same color after this hardcoded math is done. So, for disabled controls, just do

GUI.color = new Color(1,1,1,2);

and set it back when done and your control's appearance will not change. Least it's working for me in the editor so far.

As far as I know, this is hard-coded behavior and can't be changed.

You can specify how GUI elements should behave in which situations by creating your own guiskin, or modifying an existing one. Look up "GUISkin" in the help.