No, sorry, this information isn't passed by the XML parser to the XSLT processor so there is no way you can access it. The only possibility (it's pretty desperate) is to read the file using unparsed-text() (in XSLT 2.0) and parse out the relevant DTD declarations yourself.
Michael Kay
http://www.saxonica.com/
Author, XSLT 2.0 and XPath 2.0 Programmer's Reference