Are you sure? I can't find any mention of ST_GEOMETRY in the Oracle documentation (Oracle® Spatial User's Guide and Reference: http://download.oracle.com/docs/cd/B19306_01/appdev.102/b14255/toc.htm)
I know that there is an OGC-compatability package (also undocumented), which _is_ supported in HS (controlled by the the OGC_STRICT setting (see: http://www.hibernatespatial.org/hibernate-spatial-oracle/usage.html))
. It is my understanding that this is not exactly the same as what is provided in 11g under the name "SQL Multimedia Type Support".