Although I would like that too, such a feature can be scripted quite easly since there is an item "Overide contact's font" in the Plus! menu in each conversation window.
Using the
PostMessageW Windows API to send a WM_COMMAND message to the chat window with the correct command id (7408) you can trigger that menu on or off depending on what contact you're talking too [
*] (the default state of the menu when you open a chat window can be known from the registry key
FontOverrideDefault).
Throw in a nice interface to select the contacts for which you want to override their fonts (and even reusing the registry settings for the font-override preferences in Plus!) and you have the feature.
More about this in some threads in the scripting forum, iirc...
/ 
