How can you calculate the actual flow available when opening a hydrant nozzle, accounting for losses based on the orifice diameter, etc?
You can simply enter an emitter coefficient at the junction or hydrant element corresponding to the actual hydrant size. The correct emitter for your hydrant size will account for losses through the nozzle and the outflow will be dependent on the pressure available in the system. There is a discussion of this in Advanced Water Distribution Modeling and Management, pages 451-453. Typical values for a 2.5 in outlet are on the order of 165 to 185 gpm/sqrt(psi).
You can also take a look at Conventional Flushing in the Flushing tool, which allows you to essentially see results in batch for multiple, separate hydrants.
If the hydrant has been placed in-line with the distribution main pipes (lateral pipes not explicitly modeled), you can account for any losses through the lateral pipes using the fields available in the properties of the hydrant node.
Options for modeling an outflow that varies with pressure