I myself use Custom Emoticons for all Messengers default Emoticons.
MSN Messenger 7.5 allowed for modification of all your emoticons, once modified, they were saved in settings (you can back up the settings folder) located in the usual place..
I then upgraded to Live Messenger, I don't know.. one of the eariler public beta versions (not the latest) and I noticed the Default Custom Emoticons were still the ones i had set in 7.5
Now, its important to note that, once the emoticons you modified in 7.5 go onto be used in 8.0.x ... you can no longer 'modify these default emoticons but they will still be set as the ones you had chosen in 7.5, you just cannot change them any longer.
After upgrading to the current public beta, I noticed that some of my emoticons went missing, not the default ones... but a few I had set (unsure why) but.. I think the Messenger Team, changed the location and such, of where your settings are stored, and prob messed something up in the process of upgrading.
Anyhow, yes... if its possible to make a script where you can modify your default emoticons, somebody make it, or.. a patch that allows 'modification' of all emoticons, its surely possible
