First of all, I need to say that for some reasons it is not possible for me to filter objects based on id ☺
Therefore, I need to group objects based on the meta-attributes that Visio has set for them.

However, I could not get these types of features using xpath. Is it even possible to do this?

For example, I want to get the object whose v:mID is 100

<g  v:mID="100" v:index="1" v:groupContext="layer" id="g1579">

For this, I used the following rule, which unfortunately did not work:
//*[@v:mID='100']

can you post a sample visio file ? 

Yes, of course...

I've only does this a couple of times for hocr files produced by tesseract. I think you need to define the various visio namespaces in your code. 

This should find all elements which have v:mID='445'

namespaces = {'v': 'http://schemas.microsoft.com/visio/2003/SVGExtensions/'}

document = self.document

element_list = document.iterfind('//*[@v:mID="445"]', namespaces)

inkex.errormsg(element_list)

for item in element_list:
    text = item.tostring().decode('utf-8')
    inkex.errormsg(text)

this is fabulous ! I appreciate your kindness.
Is it possible to somehow use this namespace for the get() function when we have an object and for example we need to read its v:cp value . of course, I mean without manipulating the element string.

m_id = element.get('{http://schemas.microsoft.com/visio/2003/SVGExtensions/}mID')

However I don't know how to add that attribute as shorthand for use in get 

Update ! 

After asking in Chat - it's a simple as this:

inkex.NSS["v"] = "http://schemas.microsoft.com/visio/2003/SVGExtensions/"

elements = self.svg.xpath('//*')
for item in elements:
    m_id = item.get('v:mID')
    inkex.errormsg(m_id)