You can’t. Request information is related to controllers and views and should not be made available to models.
Look at Restful authentication and see how it handles that - they use attr_accessor to store data.
This is a really dumbed-down approach but you could do this:
class ModelObserved < AR::Base
rest of your code
class ModelObservedObserver < AR::Observer
do stuff with this
Then in your controller you just need to make sure you pass the request.env stuff to the model.
The other way to do it is to use cache_sweepers instead of observers. Acts_as_audited does that so you might learn something from that code.