Passing additional information to the Nova Scheduler

I've been investigating the Nova platform with the aim of creating some new scheduling algorithms.

I was hoping that someone might be able to answer a question I have about the nova architecture:

The algorithm I am attempting to construct requires the user to set a flag or option when they request the VM which is then used to influence the scheduling decision.

I'm looking for advise on what would be the best (or most accepted) way of passing a string from the user to the scheduler? My understanding is that everything flows through the API, so would it be sensible to add another parameter to the _create_instance function and have that passed on the scheduler?

Any advice would be very much appreciated.