Some of this info is already available through Finder, as follows:
Code:
set my_file_path to ""
tell application "OmniGraffle Professional 5"
set my_file_path to path of front document
end tell
tell application "Finder"
if my_file_path is not "" then
set my_file to (POSIX file my_file_path) as alias
properties of my_file
end if
end tell