Yeah, Stochastic Pi-calculus and chemical reaction networks simulators are doing this this too. They actually precisely define the probabilities of reactions based on the current amount of different molecules and their reaction rates. For instance, I used the tool called Beta Workbench (BetaWB) and hand-written simulators for modeling systems like this. It's a really cool computational model, I think.
Adding relative weights to each rule would be a simple extension to the grammar. If only integer weights were permitted it could be a simple preprocessor.
