For an mxd that has the file path as dynamic text, is it possible to extract one or more substrings of characters from either the full path or the document name, and assign the substrings to variables to be used to populate text strings elsewhere within the mxd? I'd prefer to do this within an open mxd.
You should look at the methods that are available in the os.path module. Their purpose is for parsing and manipulating file and folder path strings.
The string module allows you to search for substrings.
os.path.basename and os.path.dirname will break up the mxd name and path.