With regard to the contracted demand, there is in Brazil the concept of exceeding the value contracted. Example, if the consumer contracts 1 MW, but at some point, he demands 1,2 MW from the grid, he is subject to a penalty and he'll be charged 3x the normal demand rate in that month.
Is it possible to model this effect in Homer?
There are no explicit ways to model ratcheting clauses within HOMER today.
However, if you would like to model avoiding the high demand charge, you can limit your sale capacity (units of kW).
Here's information on sale capacity: http://www.homerenergy.com/support/docs/3.9/purchase_and_sale_capacities.html
Let me know if that doesn't answer your question,