Simple Stroke to Path extension example
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.inx and .py
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<?xml version="1.0" encoding="UTF-8"?>
<inkscape-extension xmlns="http://www.inkscape.org/namespace/inkscape/extension">
    <name>Stroke To Path</name>
    <id>inklinea.stroke_to_path</id>

    <!--  Parameters Here -->

    <effect>
        <object-type>all</object-type>
        <effects-menu>
    <!--            <submenu name="Submenu Name"/>-->
        </effects-menu>
    </effect>
    <script>
        <command location="inx" interpreter="python">stroke_to_path.py</command>
    </script>
</inkscape-extension>

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import inkex

import tempfile, os, shutil
from uuid import uuid4

def process_svg(svg, action_string):

    temp_folder = tempfile.mkdtemp()

    # Create a random filename for svg
    svg_temp_filepath = os.path.join(temp_folder, f'original_{str(uuid4())}.svg')

    with open(svg_temp_filepath, 'w') as output_file:
        svg_string = svg.tostring().decode('utf-8')
        output_file.write(svg_string)

    processed_svg_temp_filepath = os.path.join(temp_folder, f'processed_{str(uuid4())}.svg')

    my_actions = '--actions='

    export_action_string = my_actions + f'export-filename:{processed_svg_temp_filepath};{action_string}export-do;'

    # Run the command line
    cmd_selection_list = inkex.command.inkscape(svg_temp_filepath, export_action_string)

    # Replace the current document with the processed document
    with open(processed_svg_temp_filepath, 'rb') as processed_svg:
        loaded_svg = inkex.elements.load_svg(processed_svg).getroot()

    shutil.rmtree(temp_folder)
    return loaded_svg

def element_list_stroke_to_path(svg, element_list):

    action_string = ''
    for element in element_list:
        action_string = action_string + f'select-by-id:{element.get_id()};object-stroke-to-path;select-clear;'
    processed_svg = process_svg(svg, action_string)

    for element in element_list:
        element.replace_with(processed_svg.getElementById(element.get_id()))

class StrokeToPath(inkex.EffectExtension):

    def add_arguments(self, pars):
        pass
    
    def effect(self):
        pass
        
        selection_list = self.svg.selected
        if len(selection_list) < 1:
            inkex.errormsg('Please select at least one object')
            return
            
        for selection in selection_list:
            inkex.errormsg(f'{selection} {selection.get_id()}')

        element_list_stroke_to_path(self.svg, selection_list)
        
if __name__ == '__main__':
    StrokeToPath().run()
 

I should note that this script processes each element id separately in the action list.

For stroke to path, that is not really necessary. 

A better way would be to deepcopy(svg) and tag each path with a data-tag, then use a single css selector on the command call.

The data-tag can be removed with a loop later.

Windows has a 8191 character limit for the command line. 

I think the Inkscape developers have overcome this in the inkex command call code, but I usually err on the side of caution.