Grab Autokey (free) then use a script such as those posted here to lock your cursor into a region of the screen(you'll need to use autohotkey or another means to toggle that locking though).
Not a game i've played, not an autokey script i've used (i have another app that can provide cursor region locking - have used autohotkey before though, love opensource), but it was the first thing i managed to dig up via google(i wish i had looked harder last time i searched).
Might take a little learning the app, getting the script running and working a toggle, but you should have something workable with 10 - 30 mins(depending how you get on).
Hope that helps. :)
Age of Empires Online - Spreadsheet
Available free as long as AOEO lives.