Put together something for myself on Inkscape 1.1   ( I don't use earlier versions )

It uses a command call to create : 

Red dashed bounding boxes for text Elements 

Blue dashed bounding boxes for tspan Elements

Red circles for the centre points of text Elements

Blue circles for the centre points of tspan Elements.

import inkex
import random
from inkex import command, Circle, Rectangle, units, styles

def query_all_bbox(self):
    my_file_path = self.options.input_file
    ink = inkex.command.INKSCAPE_EXECUTABLE_NAME
    try:
        my_query = inkex.command.inkscape(my_file_path, '--query-all')
        my_query_items = my_query.split('\n')
    except:
        return

    my_element_list = []
    for my_query_item in my_query_items:
        my_element = my_query_item.split(',')

        # Reject any malformed query lines
        if len(my_element) > 4 and ('tspan' in my_element[0] or 'text' in my_element[0]):
            my_element_list.append(my_element)

    for element in my_element_list:

        my_element_object = self.svg.getElementById(element[0])

        if 'tspan' in element[0]:
            object_type = 'tspan'
        else:
            object_type = 'text'

        cl_bbox_x = float(element[1])
        cl_bbox_y = float(element[2])
        cl_bbox_width = float(element[3])
        cl_bbox_height = float(element[4])

        cl_bbox_center_x = (cl_bbox_x + (cl_bbox_x + cl_bbox_width)) / 2
        cl_bbox_center_y = (cl_bbox_y + (cl_bbox_y + cl_bbox_height)) / 2

        my_element_object.cl_bbox_x = cl_bbox_x
        my_element_object.cl_bbox_y = cl_bbox_y
        my_element_object.cl_bbox_width = cl_bbox_width
        my_element_object.cl_bbox_height = cl_bbox_height

        my_element_object.cl_bbox_center_x = cl_bbox_center_x
        my_element_object.cl_bbox_center_y = cl_bbox_center_y

        # Mark the centres and bounding boxes of elements

        make_outlines(self, my_element_object.cl_bbox_center_x, my_element_object.cl_bbox_center_y, my_element_object.cl_bbox_x, my_element_object.cl_bbox_y, my_element_object.cl_bbox_width, my_element_object.cl_bbox_height, object_type)




def make_outlines(self, center_x, center_y, x, y, width, height, object_type):

    conversions = {
        'in': 96.0,
        'pt': 1.3333333333333333,
        'px': 1.0,
        'mm': 3.779527559055118,
        'cm': 37.79527559055118,
        'm': 3779.527559055118,
        'km': 3779527.559055118,
        'Q': 0.94488188976378,
        'pc': 16.0,
        'yd': 3456.0,
        'ft': 1152.0,
        '': 1.0,  # Default px
    }
    found_units = self.svg.unit

    try:
        pixel_conversion_factor = conversions[found_units]
    except:
        pixel_conversion_factor = 1

    my_circle = Circle()
    my_circle.style['stroke-width'] = 1
    if object_type == 'text':
        my_circle.style['fill'] = 'red'
    else:
        my_circle.style['fill'] = 'blue'

    my_circle.style['stroke'] = 'black'
    my_circle.radius = 2
    my_circle.center = (center_x / pixel_conversion_factor, center_y / pixel_conversion_factor)
    self.svg.get_current_layer().append(my_circle)

    my_rectangle = Rectangle().new(x / pixel_conversion_factor, y / pixel_conversion_factor, width / pixel_conversion_factor, height / pixel_conversion_factor)
    my_rectangle.style['stroke-width'] = 0.5
    my_rectangle.style['fill'] = 'none'
    if object_type == 'text':
        my_rectangle.style['stroke'] = 'red'
        my_rectangle.style['stroke-dasharray'] = '1,1'
    else:
        my_rectangle.style['stroke'] = 'blue'
        my_rectangle.style['stroke-dasharray'] = '1,2'
    self.svg.get_current_layer().append(my_rectangle)










class MyFirstExtension(inkex.EffectExtension):
    def effect(self):


        query_all_bbox(self)


if __name__ == '__main__':
    MyFirstExtension().run()

Should have pointed out. 

It is not possible to get the bounding box of text or tspan elements from the extension system directly. You can have a hacky attempt using pango, but it's not accurate.

The purpose of this command call is to enable the extension system to access accurate values for the placement of text by the extension :)