this is a nice idea... however I think mIRC already have a /drawrect -e which can draw a circle eventhought in help file its pointed as a eclipse.
One thing I notice that you had use x and y as a center point(for me its confused myself and this totaly because im not good in math... i think this is the correct way to draw a circle isnt it?)
I also love to see if you can support the /draw* switches such as -r so that your script can be more versatile.