HA !

I spoke too soon ! 

Try this...

(This script sets the xTran and zTran of the selected figure's Body based on its yRotate, using the bog-standard right-angle triangle sine/cosine mathemagical stuff to move the figure in a circle. Not quite what I was trying to do, but it highlights one of my major misunderstandings about callbacks, and gives a clue as to how I should be doing it.)

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# ! Callback Test Setup Version 582

import math

# I spotted this technique in C:\Program Files\Poser Software\Poser 11\Runtime\Python\poserScripts\SampleCallbacks\muscleMag.py

# 1) Do NOT try and modify parameter B from a parameter A callback

# 2) Instead, create a callback for parameter B and use parameter A there to do the required calculation

# 3) Parameter B does NOT have to change for the parameter B callback to be called.

# INSTRUCTIONS FOR RUNNING TEST

# -----------------------------

# 1) Load any figure and ensure its body is selected

# 2) Increase the frame count from 30 to 360

# 3) Go to frame 360 and set yRotate to -360 (yRotate increases anticlockwise when viewed from above, I want this to go clockwise)

# 4) Change camera to top view and zoom out a bit, enough to see a circle of radius 1PNU around the worldorigin.

# 5) Slide the frame counter back and forth a few times - the figure should rotates about its origin

# 6) Set the frame counter back to frame 1

# 7) Run this script

# 8)Slide the frame counter back and forth a few times - the figure will rotate along a circle radius 1PNU centred at the origin.

# Callback for the "yRotate" parameter

def cheapskatesYRotateCallback(parm,value):

return value % 360 # Go round in a circle

# Callback for the "xTran" parameter

def cheapskatesXTranCallback(parm,value):

return value - (onestep*math.sin(math.radians(parmYRot.Value())))

# Callback for the "zTran" parameter

def cheapskatesZTranCallback(parm,value):

return value - (onestep*math.cos(math.radians(parmYRot.Value())))

global parmYRot

global parmXTran

global parmZTran

global onestep

body=poser.Scene().CurrentFigure().ActorByInternalName("BODY")

parmYRot=body.Parameter("yRotate")

parmXTran=body.Parameter("xTran")

parmZTran=body.Parameter("zTran")

onestep=1.0

print "Setting up the 'yRotate' callback..."

poser.Scene().CurrentActor().Parameter("yRotate").SetUpdateCallback(cheapskatesYRotateCallback)

print "Setting up the 'xTran' callback..."

poser.Scene().CurrentActor().Parameter("xTran").SetUpdateCallback(cheapskatesXTranCallback)

print "Setting up the 'zTran' callback..."

poser.Scene().CurrentActor().Parameter("zTran").SetUpdateCallback(cheapskatesZTranCallback)

print "...done."

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The 3Dcheapskate (also available in DAZ and HiveWire3D flavours) occasionally posts sensible stuff. Usually by accident.
And it usually uses Poser 11, with units set to inches. Except when it's using Poser 6 or PP2014, or when its units are set to PNU.