I am not sure if you have already worked this out, it stumped me for a while!
There is a haloint key in the lamps IPO window, but you cannot set it using the I key to insert a key - you have to do it manually.
So, select the lamp - and then change an IPO window to show the lamp IPO.
You will see an entry on the right hand side that say Haloint.
Select this with the Left MB, and then in the IPO curve window, CTRL and Left MB -this will insert a key.
You can then use the N key to show the properties of the key, and you can manually change it to what you need. it’s not too difficult as it is only values between 0 and 1 , and the relevasnt frame number.
Hope that points you in the right direction, I have only just got internet access back so haven’t had time to post an example.