I do not think that the LO Extension Center would be a good place for that. We need to keep non-LO packages off the LO Extension and Template sites.
As for the Thesaurus in LibreOffice, well you need to install a dictionary/thesaurus .oxt file as the easiest way to get one working with LO. I know there are Thesaurus files in the Ubuntu repository, but I do not know how to get LO to access them properly. There seems to be several file formats for Ubuntu's internal/repository spelling and thesaurus data files. LO must be able access them properly to use them. Since Windows does not include such files, as far as I can tell, would the developers create the coding for each type of Linux dictionary/thesaurus repository based file formats when Mac and Windows might not have them? Would these file be in all the Ubuntu, Debian, Red Hat, openSUSE, etc, based distros, or only in some of them? I would not want to be the one responsible for testing LO to see if it works with each and every distro's repository-based spelling dictionary and thesaurus files. It is much easier to just work with the ones enabled through the installing of the .oxt file based ones.
http://libreoffice-na.us/English-3.4-installs/dictionary.html is the list of all the .oxt dictionary and thesaurus files I know of, except any new ones listed in the LO Extension Center that I have not listed [yet]. There are over 200 dictionaries [last counted] listed, with over 20 localized Spanish ones. So, to be honest, I would rather deal with using a .oxt file-based thesaurus and dictionary than try to get what Ubuntu has to work with LO.
As for the question about stand-alone Thesaurus for Ubuntu, for now Artha would be the easiest route to go. Install it, then run it. You will not see it's window coming up normally. You have to do a "ctrl-alt-w" to have the window pop up for use. The package stays in memory, or partly, until you reboot [or at least as far as I know it does]. I have used it a few times though. Actually it is more than just a Thesaurus. There is a "Relatives" tab system that lists their titles called: Synonyms, Antonyms, Derivatives, Related to, Attributes, Similar, Domain, Causes, Entails, Kind of, Kinds, Part of, and Parts. What all these options will show, I have no real clue, but they are listed.