Would a script that sets display messages for instant messengers be simple or complex? After some searching, there doesn't seem to be any information about this at all.
For the sake of an example, if I had a text file of quotations, would it be possible to have the google talk display message change to a different quotation hourly?
Depends on which client you're using. As far as I know, Google's client doesn't offer any interface for plugins, but the open source instant messenger Pidgin does. I think there already is a plugin for what you want to do, but you can write your own using the documentation and examples they give you.
The complexity of writing something like this is based on how much C or Perl you know, since you can program in either of those for Pidgin. Reading code from other people's plugins, you should be able to figure out the Pidgin API.