To my knowledge the university of vienna is using XOTcl for part of their custom code for .LRN. But I guess it really depends what you need this for.
If a new package would require the use of TCL LIB, we'd just add tcllib to the installation instructions, same can happen with XOTcl or other extensions that are loadable for OpenACS, so I don't think the community limits itself to just use TCL, but does not see much need for using something else (at the moment).
Actually, I do think we should think about supporting tcllib more, make use of it and get a decent API browser for the installed TCL commands.
