Kojak…I wouldn't recommend you try that for one very good reason.
1.If they do decide to check your bike bag and dicover it filled with other stuff then you'll have no hold bag to put said items into! and then you'll be screwed.
We did put some stuff in our bike bags last year, like camelback reserviors and tools, but we still had plenty of room in our 1 hold bag should they have discoverd it! Your choice i quess but i personally wouldn't risk it for the sake of a few quid. We always book our flights as early as possible so that the total cost is as low as possible.
Our flights in September to Geneva are just under £200 for the 2 of us with 2 bike bags and 1 hold bag.