Michael,
There is not a direct one-to-one analogue to zero-span in the API. Zero-span is implemented in Spike with the I/Q streaming capabilities in the API. The I/Q data is then converted to all the plots (AM/FM/spectrum/waterfall/etc). There are examples of capturing I/Q data in the SDK. Conversion routines from I/Q to AM or spectrum are available online or made easy in environments like MATLAB.
Let us know if you have follow up questions.
Andrew