The MSD "Digital 6" for one offers a single stage retard setting for use with nitrous, as do many of their digital series ignitions.
This is one of the simpler setups which should be sufficient for what your looking to do. More advanced systema offer several stages of retarding, as well as means to verify that the nitrous has actually shut off and is used up before the ignition returns the advance.
http://www.msdignition.com/ignition_18_6520.htm