Hiya,
Yes, that should be possible.
Zest can call any standalone scripts registered with ZAP.
If your Python script can be implemented in ZAP (using Jython) then you can call that directly.
If not then implement a simple wrapper script (e.g. using Jython or JavaScript) and then call your Python script from that one.
Make sure you Python script returns there right value and your wrapper script returns the value from that script.
Cheers,
Simon