hello hm2k,
Sorry for answering you this late. I was kind of busy lately.
Your idea is good and it would be wonderful if you could do it. To answer your question, I don't know how feasable it is because I don't know anything about eggdrop. But I can tell you about amsn.
Yes indeed, amsn is written in Tcl/Tk and it's easy to learn, just refer to the manual pages at
http://tcl.tk it should be enough if you have any programming/scripting experience, even small.
Now, about interface with a "non-gui" version of aMSN.. well, we don't really have a non gui version. There is one thing though that I created, called amsn-remote and amsn-remote-CLI (contributed by a user) which allow to remote control your amsn session. The -CLI version is a Command Line Interface but it still needs a remote amsn to control (which needs to be already launched and configured and has a GUI).
Now, you could either use that to interface with amsn. Or you could create your own plugin that would allow you to interface with amsn. I think the idea of a plugin is good because it allows you to add any functionality you might want and it would be a "enable eggdrop support" as well as having your own configuration window for the plugin. Writing a plugin is easy, there's a guide in the wiki and you can follow the example of existing plugins.
About the amsn-remote. If you think it will be enough, but at some point you need something added, just look at the remote.tcl file, it contains all the accessible functions remotely. You just need to add your proc in the remote namespace and you're done. Oh and btw, your plugin could also have something like this :
proc ::remote::eggdrop_cmd { args } { ... }
which will create the "eggdrop_cmd" proc inside of the ::remote namespace, thus making it available through the remote control too. So your plugin could also easily extend the remote controller.
If you need any more help, don't hesitate!