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If your procedural takes an arbitrary list of parameters, you can use your procedural node's "rman_preShapeScript"/"rman_preShapePythonScript" attributes to run a script that will set your nodes "man_data" attribute with the appropriate arguments:

<args format="1.0">
    <page name="Parameters" open="True">
        <param name="__dsoname" type="string" default="versatileProc.so" match="-none-"/>
        <param name="data" type="string" default="" match="rman_data"/>
    </page>
    <typeTag>
        <tag value="procedural"/>
    </typeTag>
    <rfmdata classification="RenderMan/procedural"/>
</args>

If you want your preShapeScript to take the node's name as an argument, you can use string substitution tokens. For example, if your python function is called versatileProcPreShape, enter: "versatileProcPreShape('<shapepath>')" 

Example Download

An example Maya plug-in with RenderMan procedural integration: simple_procedural.zip

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