thank you for reaching out to us. We are a bit unsure about the nature of your question and if it is really SDK related. These are the Cinema 4D SDK forums, and the SDK Team only does deliver support on the Cinema 4D SDK. To retrieve customer support, i.e., for questions about the Cinema 4D app itself, please reach out to customer support.
With that being said, I do not really understand your question in the first place. I assume you want to control a MoSpline in some fashion in Python, which is possible. You however imply that you want to control the Growth parameter for a segmented MoSpline for each segment individually, by saying, "[I am] trying to do a per segment control to randomly offset the growth [...]". This is not possible as the MoSpline object has no parameters to dial in the offset on a per segment basis (at least I am not aware of any). If you can control the growth on a per segment basis individually with fields, is a question you would have to ask customer support, as we are here more API oriented.
In case you are just after some parameter access in Python, I have attached a simple Python script at the end highlighting this.
""" Creates a new MoSpline and sets some if its parameters.
To be run as a Script Manger script in the Script Manager.
"""Script entry point.
# Create a new MoSpline object.
spline = c4d.BaseList2D(440000054)
# Set its mode to turtle and growth mode to per segment.
spline[c4d.MGMOSPLINEOBJECT_MODE] = c4d.MGMOSPLINEOBJECT_MODE_TURTLE
spline[c4d.MGMOSPLINEOBJECT_GROWMODE] = c4d.MGMOSPLINEOBJECT_GROWMODE_SEPARATE
# Set the growth of all segments to 10.
spline[c4d.MGMOSPLINEOBJECT_TURTLE_GROWTH] = 10
# Insert the new MoSpline into the active document and push an update event
# to Cinema 4D.